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Basements for housing
Benefits and solutions for sustainable housing
2
Basements for housing
IntroductionThis document considers the role of basements as a viable part of the solution to sustainable housing in the UK, through consideration of their specific attributes in relation to planning policy and the Code for Sustainable Homes.
Outlining the key issues to be considered at each stage of the design
and construction process for single-storey basements in domestic
housing, guidance is additionally offered on sources of more detailed
advice.
The Code of Practice for protection of below-ground structures against
water from the ground (BS 8102) was revised and re-issued in December
2009.
About this publicationDomestic basements can aid in the creation of desirable, sustainable
homes, providing greater flexibility and adaptability of space; thus
extending the design life of the building.
Including a basement maximises usage of available land, provides a
stable construction base and improves thermal efficiency.
Modern basements offer the possibility of additional, alternative
living spaces in dry, warm, day-lit rooms with good ceiling heights and
ventilation levels. Usage potential ranges from ideal quiet areas for
home working or leisure to additional space for storage and parking.
The use of full or partial basements in housing can play a significant role
in meeting current and future needs for new homes in the UK. Basement
design supports basic sustainability principles such as longevity, durability
and adaptability, as well as providing useful space for many of the additional
requirements needed to comply with the Code for Sustainable Homes.
Definition: Basement
Throughout the document, the term ‘basement’ refers to a ‘usable part
of a building that is situated partly or entirely below ground level’, as
defined by the British Standards Institute [1].
Building regulations in England and Wales define a basement storey as
at least 1.2m below adjoining ground level [2].
Occasional reference is made in this document to partial- or semi-
basements to remind readers that usable domestic basements are likely
to include windows and doors for natural lighting and ventilation. All
walls enclosing a basement may not, therefore, be fully below ground
level; for example on sloping sites or with lowered courtyards.
This document is principally concerned with the issues associated
with new basement construction rather than existing cellars since the
latter is, by definition, limited to space for storage below ground with
less requirement for daylighting and ventilation. Much of the guidance
is, however, relevant to the conversion of old cellars into habitable
basements.
ContentsTypes of basements 4
Benefits of basements 6
Optimising development potential 9
Construction techniques 11
Design issues 15
Costs 19
Building legislation 20
Appendix and further reading 22
An example of a new build development with a basement.
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Basements for housing
The case for basements todayChanging requirements for housing
Houses with below-ground space used to be common in the UK but
their construction has declined over the last century. In mainland
Europe and North America, however, basements have continued to
be incorporated into new dwellings. In Germany, they are included in
almost 98% of housing stock.
The decline in the UK was due, in part, to changes in living requirements.
Improved transport, convenience food and the use of electric fridges
diminished the need for naturally cool areas to store food and, together
with changes in social attitude, the need for a space ‘below stairs’ for
domestic help. Until recently, reliance on cheap gas and electricity also
reduced the need for areas in the home to store fuel.
However, housing in the UK is currently undergoing a radical re-
evaluation of its design and performance, as a result of evolving
legislation and attitudes towards sustainability and energy efficiency.
Established construction techniques and space-planning solutions
for housing are being challenged and tested against a new set of
sustainable performance criteria, and basements are once more
considered relevant and attractive additions to housing.
In addition to the drive for low energy housing, there is a movement
towards increasing the size of new housing, in response to reports by
various organisations, including the Commission for Architecture and
the Built Environment (CABE), Homes and Communities Agency and
Mayor of London Office. The provision of homes that have sufficient
space to develop and grow with the requirements of a family is a
significant sustainability issue for housing.
The sustainability benefits of basements are described in the ‘benefits of
basements’ section of this document, and a summary of scoring credits
applying to basements under the Code for Sustainable Homes is given
in the Appendix.
“In Germany, and much of continental Europe, the basement provides the solution to the problem.Over there, asking for a house without a basement is like asking for a car without wheels. “
Chris Drury - Weber House, Germany, commenting on the
lack of storage space in UK housing.
Government targets for housing and limited land availability
A combination of Government targets for new housing and
limited availability of land for new construction have led to greater
consideration of increased densities and the development of ‘difficult’
plots of land, such as sloping sites or those with poor soil stability.
Planning Policy Statement 3: Housing (PPS3), underpinning delivery of the
Government’s strategic housing policy objectives, encourages increased
density of housing developments. In addition, high land prices mean
housebuilders are under increasing pressure to maximise potential
returns through efficient use of land.
The use of full or semi-basements can be a cost effective means of
increasing the density of housing developments, without reducing
amenity levels; particularly if planning requirements restrict the building
footprint or height.
Strategies for optimising residential development are explored on page
9 and 10 of this document.
Improved construction techniques and contractor warranties
The reliability and design of construction and waterproofing techniques
has significantly improved over the years; backed by many successful
examples both in the UK and abroad.
Advice regarding the detail, design and construction of basements
is well documented and available from The Basement Information
Centre (TBIC), The Concrete Centre and National House-Building
Council (NHBC), amongst others. NHBC arrangements with builders
and developers provide cover under their standard building assurance
system for the construction of basements [3].
Accreditation and indemnity schemes exist for specialist basement
contractors. Details of accredited contractors (construction and
waterproofing) may be found on The Basement Information Centre
website. In addition, various manufacturers of specialist water-resisting
concrete offer guarantees.
An accreditation and insurance scheme, administered by the Association
of Underpinning Contractors (ASUC), is available for underpinning of
existing structures; most commonly required for refurbishment or retro-
fit basements.
The correct procedures for design, soil investigation and construction
are key to achieving robust and reliable basement construction.
The various methods of construction on offer, and design issues, are
summarised in the ‘construction techniques’ and ‘design issues’ sections
of this document.
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Basements for housing
Types of basementsBasements can be broadly sub-divided into five categories, depending upon their location, time of construction relative to the main property and depth. A brief summary of the differences, and the key issues related to each, is outlined below.
Table 1: Types of basements.
Type of basement
Description Details Benefits Other comments
Refurbishment Alterations to
existing space
below ground.
May require lowering the
floor to increase head room;
underpinning adjacent walls;
improving ventilation and
lighting. Typically involves
improvements to, or new,
waterproofing system; and new
finishes and fixtures.
• Addsvalueanddesirability
of property
• Additionalusablespace
• Addressespotentialexisting
damp problems
• Providesopportunitiesfor
new activities in property e.g.
games room/gym/storage
Building regulations approval
will be required for any works.
Planning permission may be
required, depending on extent
of works and intended use. *
New garden basement
New basement
adjacent to existing
property, usually in
garden space.
New structure below ground,
with planted green roof or
terrace at garden level. Access
from main house via new
external covered staircase.
• Addsvalueanddesirability
of property
• Additionalusablespace
• Providesopportunitiesfor
new activities in property
• Potentialfornaturaldaylight
and ventilation through
roof lights
Planning and building
regulations approval required.
Likelihood of need for structural
support to existing house is
reduced as distance from house
increases.
New basement under new housing
Basement space
built as part
of a new build
development.
Arrangement of windows and
internal and external access
vary. Designed to suit current
and future use requirements,
site conditions, cost and
constructability.
• Addsvalueanddesirability
of property
• Futureadaptability
• Usablespaceforsustainable
technologies and recycling
• Potentiallyreducesfootprint
of house
• Increasesthermalperformance
No additional building
or planning regulation
requirements provided it is
included in initial application.*
Retrofit New space created
through excavation
below ground
floor of an existing
property.
Allows creation of additional
space below business or homes,
which benefit from staying in
same location.
Preservation of existing building
possible (e.g. listed building)
Underpinning works required.
• Addsvaluetoproperty
• Additionalspaceforbusiness
to develop in same location or
family to expand
• Releasespotentialofempty
property
Planning and building
regulations approval required.
Specialist work. Generally only
economically viable for high
land value properties.
Deep basements Spaces below one
storey deep.
Frequentlyusedforcarparking,
plant/services space and
storage below larger residential
development and other uses
including commercial, retail or
mixed use schemes in urban areas.
• Buildingfootprintand
development potential
optimised above ground
• Parkingandspacefor
deliveries possible
RefertoDesign and Construction
of Concrete Basements [10].
*AttimeofwritingworksmaynotbeallowableunderPermittedDevelopmentRights,dependingonindividualLocalAuthority.Thismayberevisedin
the near future. The situation should be confirmed with the Local Authority Planning Department.
Note: The Party Wall Act could apply to each type of basement. See page 21.
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Basements for housing
Some examples of basement arrangements
Figure 2: Basement with gym, shower and sauna. External and internalaccess with light well for daylight and additional ventilation.
Figure 1: Basement garage and storage. External access via the driveway.
Figure 5: Split level design with separate external access and light well. Potential for office or workshop.
Figure 6: Garden basement with internal access and roof light above.
Figure 3: Basement with games room, wine cellar and storage. With external courtyard for daylight and additional ventilation. Access via
courtyard and separate internal stair.
Figure 4: Basement with additional bedroom or annex to the property (granny flat or similar) with internal and external access.Extra light
provided via conservatory.
6
Benefits of BasementsThere are many reasons for the provision of basements in a housing development including: added value; increased development potential; occupant or purchaser attraction and sustainability. This section explains the key benefits.
Desirable and adaptable spaces
There is an evident desire for the provision of basements as part of
our housing solution in the UK, illustrated by the significant number of
basements constructed in the self-build market.
In addition, studies by the Traditional Housing Bureau [4] indicate
significant demand from home owners for more space. In the 2005 CABE
report What home buyers want: Attitudes and decision making among
consumers, basements are cited as particularly valuable in this regard.
One of the major benefits of basements is allowing the creation of a single
large space. Due to economies and method of construction, the floor above
the basement level can be created in a single span, providing flexibility in
the location of internal walls and allowing simple future alterations.
Pre-subdivision, the resultant space is typically the single largest area
in the house and, due to its location on a different level from the rest
of the house, provides opportunity for uses not always possible to
accommodate in more basic dwellings. In North America, Canada and
continental Europe, it is common for basements to serve as multi-
purpose areas, for example games or utility rooms or storage areas.
Basements can provide comfortable day-lit rooms, with natural
ventilation and external access, as an extension to the living spaces
above. They also provide the opportunity for more unique uses, such as
gyms, music rooms and swimming pools. Alternatively, basements can
simply provide practical space for games or hobby rooms, home offices
parking or storage.
Good sound insulation
Good acoustic attenuation is provided by the concrete walls
surrounding basement rooms by the earth itself and the ground floor if
it is built from concrete. Basement spaces are therefore inherently well
insulated for sound and ideal for locating noisy activities such as music
practice, home cinemas or other loud equipment that could disturb
neighbours or the rest of the house.
Conversely, the quiet nature of the space provides a peaceful place
for reading, relaxing or working; away from ground-level noise, in and
outside of the house.
Martin Grant Homes - Riverview Court development.
Developers in the UK are now looking at basements as a solution for creating spacious homes whilst using the land available efficiently. RiverviewCourtdevelopmentwasbuiltonaformer water treatment works on a flat site, besidetheRiverCam,Cambridgeshire.35homes were built with sunken patios which provide natural daylight and ventilation to the basement kitchen and dining room.
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Basements for housing
7
Basements for housing
Added value and space
The provision of additional floor space in a home clearly increases the
value of a property and the addition of a basement adds space with little
or no effect on the garden area. The costs associated with constructing a
basement are explored in the Costs section of this document.
Sustainability benefits
Including a basement in the design of domestic properties helps
developers to address many of the fundamental principles of
sustainable design, including improved energy efficiency, longevity and
adaptability and support of local employment.
Energy efficiency
Basements benefit from the surrounding ground improving their energy
efficiency. As a consequence, the amount of insulation needed to reduce
heat loss through a basement wall is less than that required on upper floor
levels [5]. The simple construction methods and minimum wall penetrations,
associated with basement construction, also lead to minimal heat loss
throughcoldbridging.StudiesbyTBICandBuildingResearchEstablishment
(BRE)highlightapotential10percentsavinginspaceheatingfora
two-storey house with a full ground basement compared with its three-
storey equivalent above ground (both having the same amount of added
insulation). The potential space heating saving rises to around 14 per cent
for a single storey property with full basement, compared to its two-storey
equivalent above ground [6].
Thermal mass
The heavyweight nature of basement construction can be utilised to
naturally regulate the internal temperature of a home and can be part
of an energy efficient strategy for controlling the temperature of the
whole house. The Met Office has projected average daily temperature
rises throughout the UK, indicating the increasing need for low energy
solutions to cool homes, which heavyweight construction - including
basements-iswellplacedtoprovide.Forfurtherinformationreferto
The Concrete Centre publications Thermal Mass Explained and Thermal
Mass for Housing. www.concretecentre.com/publications.
The thermal mass properties of concrete are optimised by omitting
insulating internal surface finishes. If insulated and waterproofed
externally, basement concrete walls will offer greater thermal mass. This
could be achieved with a fair-faced or painted finish, or alternatively
awetplasterfinish.Fair-facedconcreteofferspotentialcostand
programming benefits, by omitting subsequent use of finishing
materials and trades and associated waste produced on site.
Air tightness and mechanical ventilation
The construction of sustainable dwellings using low air permeability
and mechanical ventilation with heat recovery, such as the Passiv Haus
technique, is a means of improving the energy efficiency of the building
fabric of dwellings. This is simply provided by basements, since the
structure below ground is inherently more air tight.
The addition of a basement increases the living space in a property, without compromising the garden. Courtesy of The London Basement Company.
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Basements for housing
Basements and the Code for Sustainable HomesSince 2008, all new homes require rating under The Code for Sustainable
Homes (CSH) as part of Home Improvement Packs (HIPs). The inclusion
of a full or partial basement can provide additional credits under the
assessment criteria of the Code; for example, storage of waste, recycling,
alternative fuel supplies such as wood pellets, other equipment
associated with renewable energy or water recycling and even bikes.
In addition, basements can be used to reduce the overall size of the
building footprint, relative to the number of storeys, scoring points in the
Ecology Section of the Code. A full list of how basements can assist with
obtaining credits for Code for Sustainable Homes is found in the appendix
of this document.
Space for working from home
Basements offer the possibility of quiet, private spaces for working
from home, with the potential for direct access from the outside, and
therefore designated work-based visits or deliveries.
The solid nature of basement construction provides excellent conditions
for workshop spaces and activities that create noise, require support for
heavy equipment or require robust, cleanable surfaces.
Changing uses for basement spaces during the life of a family home
• Extra storage (recycling/chest freezers)
• DIY work shop
• Utility room
• Hobby room
• Home Gym/Sauna
• Wet room – outdoor gear
• All weather play room
• Band practice/music room
• Teenage den/bedsit
• Home office/studio
• Wine cellar
• Home cinema
• Granny annex
• Living room
Basements provide flexible multipurpose spaces through the lifetime of a home. Courtesy of The London Basement Company.
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Basements for housing
Optimising development potential
Higher density development - minimal extra heightThe use of a basement provides increased floor area in a
development without significantly increasing the apparent
size of the building; thereby improving the viability of a
development, particularly in areas where building height or
size of footprint is restricted. The potential to increase the
number of proposed dwellings in one property is boosted
if both a basement and habitable roof space are proposed;
thereby potentially creating two additional storeys in a
similar building envelope.
Utilise existing slopePartial basements on sloping sites become viable compared
to alternative substructure construction options to make up
ground levels.
Maximise site layout - build up to street boundaryBy lifting the ground floor level above street level to
create upper and lower ground floors, properties can be
constructed close to site boundaries while maintaining
privacyforoccupants.ConsiderationofBuildingRegulation
Part M requirements for access are required.
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Basements for housing
Protect amenity spaceThe use of a fully below-ground basement or partially
below-ground basement gives increased usable space
within the building footprint. Larger dwellings can therefore
be built on small sites without losing amenity space around
the buildings.
By incorporating facilities such as garages, utility rooms
or habitable space at basement level, it is possible to
reduce the footprint of a proposed property; thereby
increasing the number of houses on a given site or
along a fixed street frontage.
Better use of poor sites with poor soilWhere poor ground conditions necessitate deep foundations, the
additional cost can be mitigated by including a basement to add space
and therefore value to the proposed new properties.
Where large areas of contaminated soil are removed from site, the
viability of including a basement level is increased. If constructed before
original ground levels are reinstated, the amount of replacement ground
material is reduced and further excavation is unlikely. Back fill will need
to be compacted around the walls, but in general the programme of
construction is likely to benefit from improved access conditions.
Stable building stockBasements create a good stable structural base, capable of supporting
heavy loads above. By combining foundation design with the provision
of habitable space, the extra depth of structure provides the building
with greater ability to cope with climate change effects in the soil, such
as shrinkage or tree roots. This means buildings with basements are less
prone to movement and cracking as a result of potential future changes
in soil conditions.
Increased number of plots per hectare
Housing using a basement garage requires less street frontage,
compared to houses with garages located alongside at ground
level.
Shading indicates equivalent accommodation areas located
beside or below a dwelling, impacting on available garden
space.
9 plots possible with basement garages, compared to 8 plots on the same site.
10.0m
9.0m
7.8m
6.0m
6.0m7.8m
9.0m
7.6m
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Basements for housing
Basement constructionTypes of waterproofing protectionThere are three main methods of providing protection against ground
water for residential basements. These are defined in BS 8102 as types
A, B and C. Their application is influenced by the ground conditions and
proposed building use. The definition of terms and guidance related to their
appropriate usage, are highlighted in the 2009 revision.
Alternative approaches
The traditional method of waterproofing domestic basements in Britain
was a single barrier method (Type A) or drained protection (Type C).
Earlier materials used in basement construction have since been
developed into more durable waterproofing membranes.
Furtheralternativeapproacheshavebeendeveloped,wherebyhydrostatic
pressure on the wall structure is eliminated. These methods incorporate a
drainage blanket around the perimeter of the basement, allied to effective
drainage below the floor slab and around the building. Suitability depends
on the drainage characteristics of the ground and topography.
A combination of systems can also be an appropriate design solution.
Forexample,theapplicationofadditionalwaterproofingsystemstoa
Type B structure will improve water vapour control or provide further
protection against water ingress.
Each of these methods is viable for domestic basements in Britain,
depending upon the specifier’s preference, site conditions, the type of
development and perceived risk. Table 2 on page 12 provides a summary
of appropriate waterproofing protection for varying risks associated
with water table levels and useful additional measures to reduce risk
dependant on project particulars [7]. BS 8102 should be consulted for
further details.
Type C: drained protection – any water seeping through
external walls and floor is drained to a sump via an internal
cavity, typically created by a proprietary cavity system and
pumped or drained away.
Type A: barrier protection – reinforced concrete or block-
work with waterproofing located either externally, internally
or sandwiched.
Type B: structural integral protection - reinforced or
prestressed concrete designed through composite and
integrated details, such as water bars, to be water resistant.
Internalwaterproofing
Sandwichedwaterproofing
Externalwaterproofing
Water resistant reinforced concrete wall and slab
Slab with integral kicker
Drained cavity
Inner skin
Wall cavity
Internal block wall
Access point(s) to drainageConcrete/steel piled wall
Drainage channel
Waterstop at junction to follow wall profile
Floor slab with integral protection and/or added membrane (internal or external)
Pump
Sump formed in situ or separate drain which may be solid or perforated
May incorporate drainage channel with pipe connection to setup
Slab with kickerless construction
External or internal waterstop as required Waterstop required
at junction between wall and slab and at all construction joints. e.g Crystallisation, hydrophilic or injected waterstop
Water-resistent reinforced concrete wall and slabA non-integral kicker
should be avoided as it will require one water-stop where it adjoins the slab and another at the intersection with the wall
Internalwaterproofing
Sandwichedwaterproofing
Externalwaterproofing
Water resistant reinforced concrete wall and slab
Slab with integral kicker
Drained cavity
Inner skin
Wall cavity
Internal block wall
Access point(s) to drainageConcrete/steel piled wall
Drainage channel
Waterstop at junction to follow wall profile
Floor slab with integral protection and/or added membrane (internal or external)
Pump
Sump formed in situ or separate drain which may be solid or perforated
May incorporate drainage channel with pipe connection to setup
Slab with kickerless construction
External or internal waterstop as required Waterstop required
at junction between wall and slab and at all construction joints. e.g Crystallisation, hydrophilic or injected waterstop
Water-resistent reinforced concrete wall and slabA non-integral kicker
should be avoided as it will require one water-stop where it adjoins the slab and another at the intersection with the wall
Internalwaterproofing
Sandwichedwaterproofing
Externalwaterproofing
Water resistant reinforced concrete wall and slab
Slab with integral kicker
Drained cavity
Inner skin
Wall cavity
Internal block wall
Access point(s) to drainageConcrete/steel piled wall
Drainage channel
Waterstop at junction to follow wall profile
Floor slab with integral protection and/or added membrane (internal or external)
Pump
Sump formed in situ or separate drain which may be solid or perforated
May incorporate drainage channel with pipe connection to setup
Slab with kickerless construction
External or internal waterstop as required Waterstop required
at junction between wall and slab and at all construction joints. e.g Crystallisation, hydrophilic or injected waterstop
Water-resistent reinforced concrete wall and slabA non-integral kicker
should be avoided as it will require one water-stop where it adjoins the slab and another at the intersection with the wall
Double height concrete basement extension to existing property. Courtesy of pH+ architects.
Diagrams from The Design Guide, courtesy of TBIC, 2010.
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Basements for housing
Table 2: Types of waterproofing protection
Risk associated with water table
Water table classification*
Waterproofing protection
Type A Type B Type C
Piled Wall Reinforced concrete wall to BS EN 1992
Low
High
Low Acceptable Acceptable Acceptable Acceptable
Variable Acceptable if the “variable” classification is due to surface water. The manufacturer’s advice should be sought.
Acceptable where:a) the piled wall is directly accessible for repair and maintenance from inside the structure; orb) the piled wall is combined with a fully bonded waterproofing barrier; orc) the piled wall is faced internally with a concrete wall to BS EN 1992.
Acceptable Acceptable
High Acceptable where:a) an appropriate cementitious multi-coat render or cementitious coatings are used;b) the wall is of concrete to BS EN 1992.
Acceptable Acceptable
* The water table classifications are defined as follows:
• Low-wherethewatertableorperchedwatertableisassessedtobepermanently below the underside of the base slab, this only applies to free-draining strata.• Variable-wherethewatertablefluctuates• High-wherethewatertableorperchedwatertableisassessedtobepermanently above the underside of the base slab. Ground permeability might affect risk under a low or variable water table
Measures to reduce risk
• Usecombinedprotection• Incorporateappropriatelydesignedsub-surfacedrainageandensurethat this is maintained• Useafullybondedwaterproofingbarrier• Lowerthepermeabilityofthemainstructuralwall• Useconcretewithawaterproofingadmixture,e.gtoBSEN934• Ensurethatdischargesystems,e.gpumps,aremaintainedsothatthe system remains effective
Waterproofing barriersThis section describes in broad terms some of the options and issues
associated with the choice of waterproofing system for domestic
basementsusingTypeAbarrierprotection.Formoredetailedguidance
refer to BS 8102 and the Waterproofing Design Guide, by The Basement
Information Centre.
There are six categories of waterproofing barrier materials available. The
following table shows where they can be located, subject to the form
of supporting structure. In addition to those noted below, there are
waterproofing membranes used in Type C construction.
Water stops
Water stops are an essential part of the waterproofing design solution;
for Type B protection used at the junction of structural panels, between
walls and floors or along day-work joints for cast in situ concrete, the
principle types can be classified as:
a) Passive sections e.g. PCV water bars, located outside or within
the structure to obstruct water transmission.
Table 3: Categories of barrier protection
Categories External Sandwich Internal
Bonded sheet membranes / /
Liquid applied membranes / /
Geosynthetic (bentonite) clay liners / /
Mastic asphalt membranes / /
Cementitious crystallisation slurries
and powders
/ /
Cementitious multi-coat renders,
toppings and coatings
/ /
b) Active strips or slurries (hydrophilic or crystallization) that react with water to prevent its further progression. These are set within the section of the structure, or post-injected.c) Specialist sealing resin injected into pre-positioned permeable hoses or similar.
Design issues
Particular attention should be paid to the specification of waterproofing
systems - particularly for deep basements - relating to areas of high
water table and in soils with aggressive chemicals. An appropriate
specialist should be contacted for early advice and help on
waterproofing design.
Good design and workmanship are primary factors in achieving
waterproof construction. Key considerations are compatibility of
waterproofing systems, sealing around joints and junctions of the
waterproof membrane and, for integral structural waterproofing
systems, attention to the construction joints.
Structural design may affect the choice of waterproofing and
compatibilitybetweenthetwoisessential.Forexample,thestressand
permissible crack width of a structure is controlled by reinforcement.
In plain wall structures (i.e. not reinforced) the applied waterproof
membrane needs to be appropriate to the anticipated movement
of the structure, as the allowable movement or cracking may exceed
the strain capacity of some waterproofing membranes. This is also a
key consideration when refurbishing or extending basements, since
movement between existing and new structures must also be anticipated.
Details and construction profiles should be simple, avoiding nibs and
thickening of structure wherever possible to prevent complicated
junctions. Adequate details must be provided for each junction and
considered in three dimensions (3D) for thoroughness.
Although discontinuity with respect to waterproofing might be
acceptable - subject to careful detailing and an appropriate assessment
of risk - in practice this may not be allowed due to the need to manage
radon, methane and other ground gases and contaminants.
13
Basements for housing
Concrete constructionConcrete is the most common and appropriate material used in the
construction of new basement walls and floors. This is due in part to cost
and availability but also its inherent resistance to water, durability under
ground and ability to provide a stable structural surface for the support
of waterproofing membranes.
The method of construction chosen will depend upon consideration of
various factors including: potential repetition of construction elements;
accessibility for labour and cranes; cost; and fundamentally, the type
of construction system permitted according to water table and use, as
described in Table 2. Most forms of concrete construction can provide a
variety of wall thicknesses to suit the particular structural requirements
of each basement.
Masonry construction or concrete blockwork
Masonry construction or concrete blockwork is a traditional form
of basement construction in the UK. It can be used with Type A
waterproofing protection, for cases in which it is recommended that
render or a similar smooth, continuous layer is applied to the blockwork
face to provide continuous support to the waterproof membrane. Walls
are typically reinforced and particular care is required at corner details
and the wall slab junctions to cope with ground pressure.
Masonry walls can also be effective as internal lining to create a drained
cavity basement wall (Type C).
Cast in situ concrete
Cast in situ concrete is appropriate for all types of basement
construction. It is a common form of basement construction for
residential use, due to its relatively simple application, adaptability and
cost. In-situ concrete is often the only appropriate form of construction
for retrofit basements under existing properties, due to its relative ease
of placement on site.
As with masonry, in-situ walls are most commonly installed as reinforced
structures but can be used ‘plain’ (without reinforcement) following
guidance provided in Addendum 1- Plain masonry and plain in-situ concrete retaining walls by TBIC.
Typically, cast in situ walls are constructed with steel reinforcement
bars to control cracking in the structure, with particular attention given
to reinforcement of the corner junctions. Plain concrete walls are not
generally specified as Type B construction due to the more critical need
to control crack dimensions. Workmanship is a key issue for successful
implementation of Type B protection.
Water stops are included in the construction joints and particular
attention is required with regards to day-working joints and the
constituents of the concrete mix. Cast in situ concrete requires time to
dry out before water sensitive finishes can be applied.
Water-resisting concrete
Concrete is inherently water-resistant and robust, making it suitable for
subterranean construction. Its water resistance can be further enhanced
by the introduction of admixtures. These admixtures (hydrophobic and
pore blocking) act to reverse the capillary or ‘sucking’’ action of the tiny
capillaries on the concrete surface and to effectively block the pores
within the concrete when subjected to hydrostatic pressure. The result is a
dry concrete that protects from water ingress. Such proprietary concrete
mixes are available for this purpose from a number of specialist suppliers.
Warranties can be obtained for products and workmanship on site.
It is still possible for small levels of water vapour to pass through these
types of concrete but they are generally very low and so unlikely
to cause a problem. Additional membranes or ventilation may be
considered, depending upon site conditions, proposed use and client or
designers’ assessment of, and attitude to, risk.
Insulating concrete formwork (ICF)
ICFsystemsuseeitherlightweighttwin-walledexpandedpolystyrene
(EPS) or extruded polystyrene (XPS) in panels or blocks to create
formwork walls, for in-situ concrete walls, typically 100 or 150mm
thick. Once in place, the formwork is filled with ready mixed concrete
and, unlike conventional formwork, is left in place to act as insulation.
Forbasementconstruction,polystyreneprovidesgoodbackground
for waterproofing barriers. Care should be taken to ensure that the
specification of the waterproofing membrane and its fixing methods are
appropriate for application to polystyrene.
ICFprovidesacosteffective,simpleandinexpensivemeansforplacingcast
in situ walls; most appropriate for new build, rather than retrofit, basements.
AnewbuildbasementusingICF. An example of concrete twin wall construction.
14
Precast concrete modular units
Precast concrete units are increasingly used in Britain and
elsewhere as a form of basement construction and provide
an excellent support for waterproof membranes, either as a
tanked membrane system or as the outer wall of a drained
cavity or even as proprietary Type B system. Precast sections
can be fabricated to specific design requirements for just-in-
time delivery, providing rapid on-site construction, integrated
water bars, low site waste and high quality finishes. They
are particularly appropriate for developments potentially
benefitting from a high number of repeated standardised
elements and the use of a crane on site. The waterproofing
detail should be designed to suit the manufacturer’s
established method of joining panels.
Twin wall
This construction method is a hybrid of precast and cast in
situ concrete walls and floors. Each wall unit comprises of two
plates of precast concrete with a cavity between, linked by a
lattice of steel reinforcement and placed on site; effectively
as permanent concrete formwork. Once units and water bars
are in place the cavity is filled with ready mixed concrete to
complete the structural wall.
Twin wall systems offer all the benefits of precast concrete
described above but with the added benefit of continuous
cast in situ concrete across the whole wall and, potentially,
floor above.
Concrete piles
Piles are more commonly used for deep basement
construction, rather than domestic situations and come in
various forms, but can be useful for the creation of retaining
walls to facilitate excavation in areas of restricted access
or close to site boundaries. Secant or fair-faced contiguous
piles can effectively become the outer wall of a Type C
construction, or be faced with concrete or waterproofed to
provideTypeBorTypeAprotection.Furtherinformationon
this and other forms of retaining structures can be found
in The Concrete Centre technical publication ‘Design and
Construction of Concrete Basements’.
Concrete floors
At basement level, floors are typically cast in situ concrete.
The choice of system will be driven in part by coordination
withthewallconstruction.Floorsatgroundfloorlevelin
housing can be constructed using a variety of different
concrete construction techniques, including in-situ, block
and beam, hollow core precast units or hybrid systems.
Typically, it is possible and beneficial to span the full width of
the basement space with the floor structure. Concrete easily
exceeds the minimum building regulations requirements
for fire and imposed loads and provides excellent sound
insulation between the spaces.
14
Basements for housing
15
Basements for housing
Design principlesThe appropriate design of basements is well established and achievable,
provided design and construction guidance is implemented.
The general principle is to assess the risk of water reaching the below
ground structure and to select an appropriate form of construction,
structure and system of waterproofing to achieve the required internal
environment.
To do this the designer needs to understand the expectations of the
client, the proposed and likely future use of the basement space and its
associated performance requirements in terms of building regulations.
It is essential that an appropriate site investigation is carried out to
establish the soil and ground water conditions. Evaluation of these
factors provides the basis for selection of an appropriate construction
method, structural solution and system of waterproofing.
It is strongly advised that a three dimensional (3D) review of structure
and waterproofing is undertaken to identify and avoid any complex
geometries, which will not be readily identified from normal two-
dimensional details.
Design issues
Basement design process (simplified)1 Establish basement use; current and future flexibility
2 Site survey and exploratory works
3 Design proposals to define type of construction, water
tight class and thermal performance
4 Detailed structural design integrated with design
of waterproofing
Roles and responsibilities
Aspects of the design process are inter-related and there are likely to be a number of options available; particularly for straightforward residential properties.
Of particular importance for new-build basements is a unified approach to establishing an appropriate design solution and defining the roles and responsibilities of the design team from the outset. It was common for the design of the waterproofing system to be the responsibility of the architect however, in BS 8102: 2009 there is emphasis on including a specialist waterproofing advisor as part of the design team so that an integrated waterproofing system is created. This can be an architect or another consultant, manufacturer or supplier, provided they have the relevant expertise. An exception to this is when the construction method is classified as ‘structurally integral protection’; when it may form part of the structural engineer’s brief, a specialist waterproofing advisor may still be required.
The client should be advised of any implications related to choice of construction and waterproofing with regards to the expected building use, future flexibility and associated maintenance requirements.
Minimising risk in basement design:Initial design should consider:• Anticipatedcurrentandfutureuseofbasement
• Anticipatedcurrentandfuturegroundwaterconditions
• Orientationofbuildingrelativetogroundwater
• Currentandfuturedaylightingandventilation
requirements
• Simplifyingshapetofacilitatewaterproofing
• Locationandaccessonsitetofacilitateconstruction
• Avoidingpenetrationofwaterproofmembranefor
services where possible
Site investigations should include:• Appropriatequalitativeassessmenttoappropriatedepth
• Geotechnicalinvestigationtoindicatecurrentand
anticipated future ground water regime
• Teststoindicatesoilpropertiesandsurfaceloadingto
establish lateral earth pressures
Detailed design should consider: • Correctchoiceofconstructionandwaterproofingto
suit ground conditions and use
• Integrationofstructuralandwaterproofingdesignto
best practice recommendations
• Three-dimensionalstructuralloadsofbuilding,ground
and water pressure with attention to corners
• Accessforfuturemaintenanceandalterations
• Obtainingspecialistadviceparticularlyforhigh
water tables
Construction should include:• Supervisionandchecking(bothessential)
• Experiencedandskilledoperatives
• Instigationofconstructionwarranties
In use:• Maintenanceandoperationofdrainage,pumpsand
ventilation systems
16
Basements for housing
Table 4: Grades of basements
Grade Basement Usage Performance Level
1 Car parking; plant rooms (excluding electrical
equipment); workshops
Some seepage and damp areas tolerable, depending on the intended use*
Local drainage might be necessary to deal with seepage
2 Workshops and plant rooms requiring drier
environment (than grade 1); storage areas
No water penetration acceptable
Damp areas tolerable; ventilation might be required
3 Ventilated residential and commercial areas
including offices, restaurants etc; leisure centres
No water penetration acceptable
Ventilation, dehumidification or air conditioning necessary, appropriate to the intended use
* Seepage and damp areas for some forms of construction can be quantified by reference to industry standards, such as the ICS’s Specification for piling
and embedded retaining walls.
Basement use - current and future
It is essential that the current and proposed use of a basement space is established early in design development, in order to provide the relevant performance criteria for the subsequent choice of waterproofing system, construction method and structural design.
BS 8102 designates building uses against three grades of water tightness. These range from car parking areas, where some seepage and damp patches are tolerated, to ventilated residential and commercial areas where no water penetration is acceptable. Standards and forms of construction and waterproofing suitable for each grade of usage are provided.
The previous edition of the British Standard (still referenced in the Approved Document - Basements for Dwellings) referred to Grade 4 environments. This was omitted in the later version since the only difference from Grade 3 is the performance level related to ventilation, dehumidification or air conditions. BS 5454 provides specific guidance related to the storage of exhibition or archival documents.
A Grade 2 environment may be acceptable for permanent workshops or garages. However, since usage may change, it is better to construct a basement to a Grade 3 environment than to upgrade it later. In a high risk situation, the client and designer may wish to opt for additional waterproofing or vapour control.
Site investigationThe location and potential fluctuation of the water table is the key factor
effecting basement design and construction. High water tables present
the greatest risk for a basement and must therefore be identified at an
early stage in the design. A watercourse or water table that rises and
falls, and the potential for a perched water table, must also be identified.
A high water table refers to, by definition, groundwater level consistently
above the level of the basement floor. A permanently low water table
involves a water table consistently below the level of the basement floor.
A variable water table refers to levels varying between the two extremes.
The installation of drainage systems can artificially lower the water
table but is not always beneficial due to potential detrimental effects on
neighbouring properties.
The draining ability of the soil and existence of contaminants can effect
the choice of concrete construction and waterproofing method, as will
the location of nearby drains and an assessment of the likelihood of
their flooding.
Typical factors to be assessed in site investigation
• Existenceofwatercourseorseasonalposition
of water table
• Topographyoflandanddirectionofground
water movement
• Locationofdrainsandlanddrains
• Soiltypeandconditions
• Movementrisks-potentialsubsidence
• Presenceofnaturalgasese.g.radon/methane
• Evidenceofgroundcontaminants
• Boundaryconditions
Mirrors facing and adjacent to, window openings can significantly increase the perceived light levels. Courtesy of The London Basement Company.
17
Basements for housing
Orientation and site layout
The shape and orientation of a building should be considered because
of the potential to dam the flow of ground water and the resultant
build up of hydrostatic pressure. If unavoidable, additional subground
drainage may need to be provided to discharge the water elsewhere.
The form of construction of a basement and its cost will be influenced by
the proximity of its walls to existing boundaries and adjacent buildings.
The installation of external waterproofing and insulation, for example,
requires sufficient space around the outside of the basement walls to
provide a safe working area and may require temporary shoring.
Fornewbuildconstructionsintight-usuallyurban-plots,permanent
underpinning of adjacent boundary walls or properties can allow
valuable additional basement floor areas, but is expensive. An alternative
is to install sheet piling to contain the ground supporting the structure
while the new basement is under construction.
Flowofgroundwater
Flowofgroundwater
Plan form of building avoids possibility of damming the flow of ground water
Provide subground drain discharging to a suitable outfall to alleviate hydrostatic
pressure where necessary
Effect of building orientation on flow of ground water
Daylight The need to provide daylighting and comply with building regulation
requirements for ventilation of habitable rooms will generally be met by
incorporating openable windows in the same manner as above ground.
This may entail adjusting the external ground levels in partially below-
ground basements, and would mean forming open areas for windows in
fully below-ground basements.
A primary factor in improving the quality of a room in a basement is the
provision of natural light. Inclusion of glazed windows or doors provides
greater possibility of future adaptation and uses, as well as sustainability
benefits by reducing dependence upon artificial lighting.
There are many techniques for improving the level of natural daylight and
ventilation in basement spaces; determined by various factors including
the proposed use of the space, proximity to boundary and plot size.
Solutions for habitable spaces include simple direct lighting through
windows, glazed doors or roof lights. Other supplementary solutions
include the use of sun pipes or use of borrowed light with mirrors,
glazed floors or stairwells.
Daylighting techniques
• Partiallysunkenlightwellsandwindows.
• Fulldepthexternalspaceswithglazeddoorsproviding
separate private amenity space and potential access to
the garden or alternative entrance from the street.
• Sunpipesandpavementlightsaresuitablefor
basement spaces extending beyond the footprint of
the building above. They provide permanent natural
lighting with additional security but limited views.
• Glazedrooflightscanwashspaceswithnaturallight
and provide sky views and natural ventilation if
openable.
• Mirrorsfacingandadjacentto,windowopeningscan
significantly increase the perceived light levels and
provide depth of field. Light and/or polished surfaces
will generally improve the sense of space and daylight
levels in a room.
• Glazedfloors,particularlybelowupperfloorrooflights
or windows, can be useful additional sources of light
but will require fire-rated glazing to maintain fire
compartmentation between floors.
• Lightfromupstairsroomscanbrightenlowerground
floor spaces via the stairwell. This arrangement will
depend upon the specific fire arrangements of
individual properties and may require an upgraded
fire resistance or detection system.
Light from the upstairs room can brighten lower ground floor spaces via the stair-well. Image courtesy of Loates Taylor Shannon architects, Paul Avis photography.
18
Basements for housing
Drainage It is advisable that drainage, or any service connections, should not be
made through the basement retaining walls. Even if invert levels are
lower than the outlet point, it is best to provide an up-and-over system,
due to the potential for reverse flow.
The location of utility spaces and bathrooms in basements has
been facilitated by readily available pumped drainage systems and
macerators. Consideration should be given to easy access for future
maintenance and replacement.
Structural designCoordination of the structural design with the construction and
waterproofing system is essential. At a domestic scale, the correct masonry
construction to back up Type A barrier protection may be determined
from Approved Document – Basement for Dwellings.Reinforcedconcrete
walls and basement slabs, especially those used as Type B structurally
integral protection, will require detailed structural calculations. The
calculations take into account the ground, groundwater, the construction
method and the required performance to determine the amounts of
reinforcement required in the sections and specification of the concrete.
Where piling is required, for instance as part of a Type C protection
solution, then a more specialist design will be required and that must be
integrated into the overall structural design.
With respect to Type A protection, simple design – i.e. with limited
protrusions and corners – will facilitate the installation of waterproofing
membranes. Drainage and granular fill in front of the wall will minimise
build up of hydrostatic pressure. Avoid in-plan inverted corners that face
uphill – they can trap groundwater.
Foradviceonthestructuraldesignofbasements,seeApproved Document
– Basement for Dwellings [9], or, for larger basements, see Design and Construction of Concrete Basements [10].
Ventilation Building regulations require the provision of ventilation to all basements
(heated or unheated) to adequately control moisture vapour, be it
generated internally or brought through from the structure. Cross-
ventilation or passive stack ventilation are the most effective forms of
natural ventilation although continuous mechanical ventilation may be
required depending upon proposed use and internal arrangement of
rooms.
Forspaceswithanticipatedhighlevelsofhumidity,suchasutilityrooms,
bathrooms or gyms, mechanical ventilation is essential.
Ventilation should be directly applied to exposed external walls where
possible i.e. not through the basement retaining walls. Stack ventilation
(i.e. ventilation through a vertical vent duct) or mechanical ventilation
which can be the preferred method of providing natural cross-
ventilation, provided it does not compromise the fire compartmentation
strategy of the development. This can be effectively provided by
the staircase linking basement and ground levels, provided no fire
separating doors are required. See the Building Legislation section of this
document for more details.
Passive stack ventor mechanical vent
Ventilationduct(s)
Basement ventilation
Natural ventilation and daylight provided with open two-storey design. Double height basement courtesy of pH+ architects.
The flow of air through a basement using natural
cross-ventilation.
The flow of air through a basement using passive stack or
mechanical ventilation.
Courtesy of TBIC 2004 [8]
19
Basements for housing
Analysis of the costs of constructing new domestic basements has been carried out by TBIC in 2005 and updated in 2010 [11]. The study provides approximate construction costs for basements based on a variety of parameters, including flat and sloping sites, full and partial basements, and in-situ concrete and masonry construction. The calculations are based on two-storey detached, semi-detached and terraced houses, with varying widths of frontage.
The schematic design of a two-storey detached dwelling of 129m2 is illustrated below, along with a similar area of house, designed over three storeys, one of which is a basement. The cost model exercise by TBIC concluded that building the three-storey version with a basement fully below ground, only cost an additional 3.8 per cent to construct and is even 0.8 per cent lower if constructed as a partial basement. Offset against the saving in land value through the reduced plot size, or the potential additional return from development of more plots on the same site, the cost exercise illustrates how basements can be a viable option for increasing profitable development, particularly in areas with high land values.
Costs The cost of a basement, and its viability for construction as part of any development, will be determined by a number of factors including, most significantly, land value. Previous examples have illustrated how the inclusion of a partial or full basement can increase the potential floor area of a single dwelling and density of a whole development, thereby yielding higher returns.
Table 5: Extract summary of costs for basement construction as a percentage of construction costs [11].
Groundworks Fitting-out
Fullyfinished 18-34% 29-47%
Empty Shell 27-44% 10-19%
The empty shell specification basement An idea resulting from the cost analysis is that a basement, or semi-basement, completed to a very basic, or empty shell space specification, can yield even greater potential profit margins for housebuilders, whilst offering a reduced cost for the purchaser.
This is an attractive proposition, since the basement offers an economic
and realistic way of creating a large single room, with the flexibility
for future adaptation to suit the lifestyles and requirements of the
occupants.
The concept of providing spaces, or a blank canvas, for residents to
adapt to their specific needs sits well with the concept of design based
on resource efficiency and minimising waste to landfill.
Elemental breakdown of constructionFactorsaffectingthecostofbasementconstructionincludeground
conditions such as excavation costs, type of waterproofing system and
access for construction.
The TBIC cost analysis [11] provides an elemental breakdown of the
range of costs associated with basement construction and summarises
the varying influencing factors including type of basement, construction
type, plan form and sloping site.
Ground works associated with basement construction can amount
to between 18 and 44 per cent of overall construction costs. There
is potential scope for cost savings to be made, such as retention of
excavated soil on site for landscaping, or adoption of an empty shell
specification leading to a reduction in fitting-out costs.
Plot area264.5sqm
129sqm house with habitable basement plus garage at the side
129sqm house without a basement requires approximately 26% more land
3.0m increase in size width
Plot area332.9sqm
11.6
14.6
The schematic design of a two-storey detached dwelling of 129m2.
22.8
20
Basements for housing
Building regulationsBuilding regulation approval is required for the construction, adaptation and extension of all basements. Comprehensive guidance on all building regulations related to basement construction is provided by the Approved Document - Basements for Dwellings produced by TBIC.
When planning new basements for housing, particular attention is required to the provision of fire separation between the basement and ground floor, a fire escape from the basement and disabled access and entrance.
Below is a summary of the issues related to basements under two-storey houses with typical floor to ceiling height. Designers should consult the relevant approved documents to check requirements related to their specific design and for other housing types.
Fire resistance and separation
The basement, as defined above, is not counted when assessing the numbers of storeys for fire resistance and means of escape. Typically, for a two-storey house over a basement, 30 minutes fire resistance is required for the structure, increasing to 60 minutes where the number of storeys is four or more. Both requirements are easily exceeded using concrete.
Fireseparationbetweenthebasementandupperstoreysisrequirediftheheight of the top floor is more than 4.5 metres above the lowest external ground level. This situation is only likely to occur in two-storey dwellings if the basement floor level is less than 1.2m below the external ground level, or located on a very sloping site. The 30-minute separation required can be simply and cost effectively achieved using concrete.
The walls and floor between garage and house requires 30 minutes fire separation which also applies if located in a basement.
Ground floor flats or maisonettes with a basement level and direct main entrances require no fire separation over and above typical fire separation between apartments. Since concrete floor construction can easily provide the fire and acoustic separation needed for a separating floor, it can be possible to convert basements into separate dwellings, provided all the necessary fire escapes and ventilation etc. are provided, where such floors are utilised.
Means of escape
Habitable rooms in basements require a safe means of leaving the building. This could be provided by the main stair of the house, provided it is protected and is connected to a final exit. Alternatively, escape can be provided by an additional stair, leading to an alternative final exit. The stair can be internal, but more commonly external. Escape through windows is also permissible if designed to permit escape as defined by the building regulations. The last two options offer cost effective solutions, particularly in terms of optimising usable space, provided the external stair is positioned away from other windows.
It is worth noting that non-habitable rooms, such as kitchens, utility rooms and bathrooms can be classed as inner-rooms and, depending upon the layout, may not require separate means of escape.
It is permissible to exit into gardens or courtyards, provided they have an exit to a place of safety or are at least as long as the height of the house.
Health and SafetyAs with all forms of construction, consideration of health and safety
issues is required at all stages of design and construction. Particular
issues related to the construction of basements depend upon the exact
nature of the work, but may include working in confined spaces, falls
from height, temporary stability and craning of large structural elements.
Planning permission Currently, planning permission is required for the construction and
extension of basements, even when not visible above ground level. At
the time of writing, the extension of a property below ground is not
directly covered by permitted development rights but submissions have
been made to address this apparent anomaly.
A detailed analysis of the role of basements within the planning
guidelines of the UK has been produced by TBIC and is published on their
website as The Hidden Potential. Basements: a planning review document.
Size of development
While planning approval is required for the construction of a basement,
often the size of the proposed construction below ground is less
contentious than an over-ground structure. This is particularly useful
for increasing the proposed floor area of an existing or new property
in areas with strict planning policy controlling the construction
of new buildings, such as a National Park or Conservation area.
In-fill development in urban settings can also benefit from the
accommodation and value added by inclusion of a basement.
Building legislation
NewForestHouse,designedbyPerringArchitectureandDesign. Photographer:NigelRigden.
21
Basements for housing
New structure is less than six metres away and lower than a line drawn downwards at 45o from the bottom of the neighbours’ foundation.
Excavation and construction of foundations and basement walls within three metres of an adjacent building or structure owned by others.
Alowenergyhouse,recentlyconstructedintheNewForestNational
Park was limited above ground to the size of the original existing
single-storey structures on site. Development of the three-bedroom
family home was possible through the construction of a large basement,
containing study area, two double bedrooms, wine storage and plant
area, and a large library and TV room.
Increased density
As described in section ‘Optimising potential development’, the inclusion
of a basement level can assist in obtaining planning permission by
raising the density of a development through increasing the number of
homes without reducing the amenity levels.
Flood risk areas
There is a resistance, through planning controls and the insurance
industry, to build houses on areas prone to flooding. The provision of
any habitable rooms in basements in flood risk areas is generally not
supported by planning legislation but can be feasible if addressed
directly.Forexample,theprovisionofanescapestairtoanareaabove
the flood risk level could be an acceptable solution, rendering the
proposed development feasible with basements.
The construction of concrete ground structures or sacrificial basements
is a recognised solution for construction in areas of high flood risk. The
habitable spaces are raised a minimum of 600mm above the level of
design flood risk, while the basement area can provide additional non-
habitable storage space. Concrete is a flood resilient material and the
design and construction of the basement and ground floor can deliver
‘best practice’ both in terms of water-entry prevention to the habitable
areas and recovery from the effects of flooding.
FloatingconcretebasementshavebeenpioneeredintheNetherlands,
where 48 floating homes have been constructed in Maasbommel on the
banks of the Maas, by Dura Vermeer [2].
Party Wall ActThe Party Wall Act exists to protect the concerns of neighbouring
landowners and to facilitate an agreements between them with regards
to construction works. It will most likely be necessary to issue a Party
Wall Notice, as required by the Act, if a basement is being constructed or
extended. The diagrams below show the summary of criteria for serving
Party Wall Notice.
Less than 6m Less than 3m
Adjoining Owner
Building owners excavation
AdjoiningOwner
Building owners excavation
45o
Summary of criteria for serving Party Wall Notice under the Party Wall Act 1996 [13].
22
Basements for housing
Section Benefit potential through basements
Associated credits
The role of the basement
Energy/CO
2
Drying space 1 Potential space to house a permanent fixture for four to six linear metres of drying space, where external
options are not practical or in addition to external options for use during inclement weather. Suitable
ventilation is required to comply with Building Regulations Approved Document F Ventilation and is equivalent
to requirements applying to a bathroom or utility room.
Bicycle storage 2 Space for secure, dry storage of bicycles. Direct access to a public right of way is required, either via stairs at
the front of the house or via the garden.
Home office 1 Ideal space to accommodate the home office requirement of a minimum 1.8m wall length to allow for a
desk, chair and filing cabinet. The office would need a window with an opening casement window of 0.5m2
in order to provide ventilation and have a daylight factor of at least 1.5%. (This provision more than satisfies
the requirement for an alternative means of escape as defined by the building regulations). The inclusion of
a home office in the basement will influence the daylighting factor for the overall dwelling under the Health
& wellbeing section.
Fuelstorage None directly.
Supports potential
up to 2 points.
A basement can also provide storage for biomass materials. While this does not attract points directly, it
supports the use of biomass heaters and combined heating and power (CHP) plants which help score points
under the Energy and Efficiency calculation.
Materials The environmental impact of the ground floor of the basement would be assessed on the elements
contained in the BRE Green Guide – Domestic Ground Floor Construction. At the time of writing there is no
Green Guide rating for a ground floor designed as a basement. The CSH is likely to need to make an individual
assessment of the specific construction. The floor at ground level in the dwelling (i.e. between basement
andfirstfloors)wouldbeassessedasanUpperFloorConstructionintheBREGreenGuide.Basement
walls represent the substructure of the dwellings and are currently not considered in the CSH assessment of
environmental impacts. The external walls above the basement would be assessed against the External Wall
ConstructionelementscontainedintheBREGreenGuide.Anyinternalwallsorseparatingwallswouldbe
assessedagainsttherelevantbuildingelementsintheBREGreenGuide.
Surface water run-off
1 The inclusion of sacrificial basements in houses with a medium to high level of flooding risk could support
gaining an additional point. The basement raises the ground above the design flood level, while providing
additional non-habitable storage space below.
Waste Space for waste
storage
4 In order to obtain credits the facilities need to be adjacent to the kitchen and positioned for disabled access.
Health and well-being
Daylighting 3 To maximise the number of points available, this would require all living rooms, dining rooms or studies that
may be located in a basement to also have a daylighting factor of 1.5%. If a kitchen is located in the basement,
this must have a daylight factor of 2%. To gain additional points, these rooms would also require 80% of the
working plane in each room to receive direct light from the sky.
Sound insulation 3 or 4 A basement in a detached house would score maximum points in this area [4]. Basements provide excellent
sound insulation. Where a basement contains separating walls between dwellings, these can be built to existing
RobustDetailsspecificationswheretheappropriateconcrete/masonrywallconstructionwillallowthehighest
score(threecredits)currentlyavailableforadjacentdwellings.FulluseofRobustDetailingcreditsdepends,
however, upon the external wall construction and flanking conditions. Solid external concrete walls can provide
goodacousticinsulation,butatthetimeofwritingarenotincludedasaRobustdetail.
Private space 1 External courtyards at basement level count as private external space.
Ecology Optimise foot print 2 A basement can increase the footprint ratio of the net internal floor area over the net ground floor of most
standard design houses to achieve at least 2.5:1 and often 3:1. The latter allows maximum points to be
scored under the Code.
Use of basements and potential sustainable homes credits
ForfurtherinformationontheCodeforSustainableHomesandhowtouseconcreteandmasonryaspartofthesolution,refertoEnergy and CO2 –
masonry solutions and Concrete and the Code for Sustainable homes, both available at www.concretecentre.com/publications.
Appendix
Basements and credits scored under Code for Sustainable Homes
23
Basements for housing
References1. BRITISHSTANDARDSINSTITUTION.BS6100-1(2004) Building and Civil engineering - Vocabulary General Terms. London, BSI, 2009 pp. 16
2. CLG Approved Document B (fire safety)- Volume 1: Dwelling Houses (2006 Edition). Appendix E Definitions. London, HMSO, 2006 pp. 83
3. NHBCFOUNDATIONRisks in domestic basement construction NF4.NHBCFoundation,Amersham,2007pp.14
4. TRADITIONALHOUSINGBUREAUAttitides towards house construction - MORI survey. 1994 (pp. 25) 1999 (pp. 30) and 2001 (pp. 30)
5. THEBASEMENTINFORMATIONCENTRE:Approved Document: Basements for Dwellings. Section 5 (update pending) TBIC, Blackwater, 2010 (ref TBIC/001)
6. THEBASEMENTINFORMATIONCENTREThermal Performance of houses with basements
(BasedontheRegulationsandSAPin-placeatthetimeofthispublication).TBIC,Blackwater,2010.pp.24(Ref:TBIC/005)
7. BRITISHSTANDARDSINSTITUTIONBS8102(2009)Code of practice for the protection of structures against water from the ground pp. 38
8. THEBASEMENTINFORMATIONCENTREApproved Document: Basements for Dwellings,TBIC,Blackwater,2005pp.67-68(RefTBIC/001)
9. THEBASEMENTINFORMATIONCENTREApproved Document: Basements for Dwellings, TBIC,Blackwater,2005pp.67-68(RefTBIC/001)
10. NARAYANANRS&GOODCHILDCH,DesignandConstructionofConcreteBasements,MPA-TheConcreteCentre,due2010
11. THEBASEMENTINFORMATIONCENTRE,CoststudyofHouseswithBasements,TBIC,Blackwater,2010(pending)
12. Innovation and Research Focus – Issue 65 May 2006, pp.3
13. CLGThePartyWalletcAct1996:explanatorybooklet02BR008622004pp.18
Further reading BS 8102: Code of practice for the protection of below ground structures against water from the ground, revised and re-issued in 2009, provides
guidance on methods of dealing with, and preventing the entry of water from, surrounding ground into a building below-ground level for all below
ground structures.
Basement waterproofing: Design Guide and Basement Waterproofing: Site Guide by the former BCA, offers comprehensive basic guidance
on design, use and application of different water-resisting methods and systems. The Design Guide is being revised for issue by TBIC, with support
from The Concrete Centre, 2010.
TheCIRIAGuide:Water-resisting basement construction - a guide – safeguarding new and existing basements against water and dampness, (Report139)providesadditionalcomprehensiveguidance,withausefulsummaryprovidedbyReport 140.
Approved Document – Basements for dwellings brings into one document all of the relevant building regulations for dwellings that are affected by
the inclusion of a basement and is supplemented by Approved Document - Basements for dwellings. Addendum 1 – Plain masonry and plain in-situ concrete retaining walls.
British Board of Agrément certificates are available for some water membrane products, which are not covered by the British Standards for asphalt
or bituminous felt and for basement tanking systems.
Design and Construction of Concrete Basements will provide comprehensive guidance on the design issues for the design of deep
basements, focusing on structural calculations. To be published by MPA - The Concrete Centre in 2010.
IHSBREPress.Good Building Guide 72 , Parts 1 and 2. September 2007 are short publications providing some practical guidance on a range
of issues associated with basement design and construction, some replicating information from the Approved Document Basements for Dwelling.
All advice or information from MPA -The Concrete Centre is intended only for use in the UK by those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted by Mineral Products Association oritssubcontractors,suppliersoradvisors.ReadersshouldnotethatthepublicationsfromMPA-TheConcreteCentrearesubjecttorevisionfromtimetotimeandshouldthereforeensure that they are in possession of the latest version.
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