Basements for housing
Including a basement in residential developments helps maximises usage of available land, offers added value, more space and accommodation with additional performance benefits. Refurbishment or alterations to existing spaces below ground are also an effective way of providing additional habitable space to a property. It is possible to retrofit a basement under or adjacent to an existing property.
A basement story is defined within the Building Regulations of England and Wales as at least 1.2m below adjoining ground level. This aspect has implications for natural lighting and ventilation. For much deeper structures there are several other options to cover there two important issues.
Benefits of domestic basements include:
- Added value.
- Additional habitable or useful space.
- Increased floor area in locations with limited development potential above ground.
- Creation of flexible, adaptable space, due to creation of single span structure.
- Spaces with extra sound insulation, expanding range of potential use, including home working, home cinema or wine cellar!
- Spaces with good thermal mass and potential energy efficiency and cooling.
- Good stable structural base, capable of supporting heavy loads above.
- Resilience to local environmental effects in the soil, such as shrinkage or tree roots.
- Flood resilience solution, by raising habitable spaces above flood level while providing useful non-habitable storage space below.
The appropriate design of basements is well established and achievable providing design and construction guidance is implemented.
Basement design process simplified:
- Establish current and anticipated future use of basement.
- Site survey and soils analysis including exploratory works.
- Design proposals to define type of construction water tight class, ventilation and thermal performances.
- Detailed structural design integrated with design of waterproofing/drainage.
- Utilities entry, Earth Pipes etc and construction waterproofing, detailing etc
Aspects of the design process are interrelated and a unified approach should be established defining roles and responsibilities of the design team. It is advisable to avoid complex geometries to facilitate waterproofing and to produce a three dimensional review of structure and waterproofing.
Reinforced Concrete 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 adaptability, inherent resistance to water, durability underground and ability to provide a stable structural surface for the support of waterproofing membranes. Options include masonry or blockwork, cast insitu, precast concrete, twinwall and insulating concrete formwork (ICF). The graphic shown has an insulated layer on the outside of the basement wall - however most practical is Insulated Concrete Formwork [ICF]
The method of concrete construction chosen will be influenced by the type of waterproofing protection permitted according to the water table situation and proposed basement use as described in BS8102 Code of Practice for the protection of below ground structures against water from the ground.
For further guidance on the benefits of basements for housing and principles of construction and design refer to Basements for Housing.
The publication ‘Concrete Basements: Guidance on the design and construction of in-situ concrete basement structures’ provides comprehensive guidance on the design issues for the design of deep basements, focusing on structural calculations.
Other useful resources: