Digging Down for Extra Space
A basement is a worthwhile consideration for a new home. It provides extra living space without adding to the visible volume of a building, often a bone of contention with the planners.
Making the most its site, this new house by German company Baufritz has a basement built into the slope of the plot. The completed house can be seen below
Below-ground rooms have a long association with homes in the UK. Cellars for storing coal were common in Victorian and Edwardian houses, while grander period properties had large basements to store food, wine – and even a team of servants. Both servants and cellars went out of fashion in the last century, the latter largely the result of our declining reliance on coal.
Basements, however, continue to be popular in northern Europe, particularly Germany, Austria and Switzerland, where sloping building sites are common. These plots are ideal for building into the bank to provide anything from a gym or plant room to bedrooms and even home cinemas. German housebuilders such as Huf Haus and Baufritz are synonymous with houses that have large basements, effectively providing an additonal storey of living space without affecting the ridge height of the building.
In London, basements have enjoyed a renaissance due to the high price of land in desirable areas. The number and alarming size of some of the multi-storey subterrannean extensions has prompted some boroughs to clamp down on the practice. Elsewhere, though, a reasonable-sized basement is considered a good use of space, especially where local planning restricts ridge height in areas where a new home may be surrounded by bungalows. A basement can also be a practical and economical solution to problems associated with building on a sloping site. At the very least, this void can provide a garage/workshop/plant room and can often create much more with a real wow factor such as a games room, office, playroom, guest suite, or even a complete annexe.
Alterations to existing space below ground. This may require lowering the floor to increase head room; underpinning adjacent walls; improving ventilation and lighting. It typically involves improvements to, or new, waterproofing system; and new finishes and fixtures.
Benefits • Adds value and desirability of property. • Additional usable space. • Addresses potential existing damp problems. • Provides opportunities for new activities in property.
Building Regulations approval will be required for any works. Planning permission may be required, depending on extent of works and extended use.
New garden basement
New basement adjacent to existing property, usually in a garden space and below ground, with planted green roof or terrace at garden level. Access from main house via new external covered staircase.
Benefits • Adds value and desirability of property. • Additional usable space. • Provides opportunities for new activities in property. • Potential for natural daylight, and ventilation.
Planning and Building Regulations approval required. The likelihood of the need for structural support to existing house is reduced as distance from the house increases.
New house basement
Basement space built as part of a new build. Arrangements of windows and internal and external access vary. Designed to suit current and future use requirements, site conditions, cost, and constructability.
Benefits • Adds value and desirability of property. • Future adaptability. • Usable space for sustainable technologies and recycling. • Potentially reduces footprint of house.
No additional building or planning regulation requirements, provided it is included in the initial application.
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 the same location. Preservation of existing building possible (eg listed building). Underpinning works required.
Benefits • Additional space for business to develop in same location or family to expand. • Releases potential of empty property.
Planning and Building Regulations approval required. Specialist work. Generally only economically viable for high land value properties.
The Technical Aspects
From the outset, your architect/designer should be familiar with basement design and construction. If not, they should work with a structural engineer or waterproofing specialist who is. The level of required watertightness will depend on the function of the basement – some damp may be okay in a garage but not in a living area.
Sites need to be assessed for hazardous gases.
Gases are also a health hazard so the site conditions will need to be assessed at an early stage for the presence of radon, methane, carbon dioxide, VOCs, and even mercury. Safeguarding against the hazards posed by these gases will usually require more than simply specifying a gas membrane, and adequate ventilation is essential.
Building Regulations requirements for ventilation will generally be met by incorporating windows. This may entail adjusting the external ground levels in partially below-ground basements, and may mean forming open areas for windows in fully below-ground basements.
Windows or glazed doors also allow in plenty of natural light, meaning the space has more potential for future adaptation and uses, as well as requiring less artificial lighting.
Sun pipes, mirrors and glazed floors or stairwells can improve light levels.
A light well is the most obvious way of naturally illuminating an area to be used as a living space. This is less important if it is going to be a cinema or plant room, but bear in mind that a basement is likely to be used for different purposes over the lifetime of a property.
A well-designed light well, complemented with tall windows or even doors at basement level, will also totally change the feel of the space, making it just as impressive as an upper-floor room.
Externally, light wells can either be open and protected with railings, or covered using horizontal grilles, glass panels or a combination of both. It is usual for the inside of light wells to be painted white and recommended that doors are fitted with glass panels if possible. The light well will also provide a means of escape in case of fire.
For security, a locking mechanism should be installed on the grille covering the light well. Glass panels can also be fitted within the ceiling above to access additional light from the ground floor.
Methods of Construction
The most suitable method of construction will depend upon various factors such as accessibility for labour and cranes, budget, and the type of construction system permitted according to water table and use.
Concrete is the most common and appropriate material for walls and floors, due to cost and availability as well as durability underground.
Cast in-situ concrete is a popular choice because of its relatively simple application, adaptability and cost – it is often the only appropriate form of construction for retrofit basements under existing properties because of its relative ease of placement on site.
Masonry construction or concrete blockwork can be used with suitable waterproofing protection, and walls are typically reinforced, with special care required at corner details and the wall slab junctions to cope with ground pressure.
Insulating Concrete Framework (ICF) systems use either lightweight twin-walled expanded polystyrene (EPS) or extruded polystyrene (XPS) and usually simply lock together to create a suitable form for the structural walls of a building. As with in-situ formed concrete walls and floors, reinforcing steel bars are usually placed inside the forms before concrete is poured. ICF is left in place to act as insulation and the polystyrene provides a good background for appropriate waterproofing barriers.
The basement may also require the installation of a sump and pump, which have to be sized to suit the project.
It is possible to add a basement under an extension or to a garden, but it may require the existing property to be underpinned unless the basement is kept a sufficient distance away, with the superstructure bridging back to the existing property.
There are three main methods of waterproofing a basement.
Barrier systems, such as tanking
Structurally integral systems (which are reliant upon the design and construction of the basement as an integral shell)
Drained protection, where any water is drained to a sump via an internal cavity.
A combination of these three systems can also work well.
All products used should have a British Board of Agrément accreditation with an insurance-backed guarantee.
Appropriate heating and ventilation will help prevent any condensation issues.
The location of services such as water, gas and drains will help determine the location of any sanitaryware.
Basements are naturally well insulated by the earth that surrounds them but will require additional insulation to meet Building Regulations.
This usually involves adding a thick layer of insulation over the damp-proof membrane. When underfloor heating is to be installed the insulation is laid under the pipes.
Plasterboard walls can be fitted around the perimeter of the core but, as it is essential that the damp-proof membrane is not punctured, these are erected on a metal frame that does not touch the membrane.
Timber or metal stud wall partitions can be used to divide up the floor space into individual rooms, if required, and will also be used to carry services. If tiles or wooden flooring are to be fitted onto the screeded floor a secondary membrane will create a cushioned effect.
Ground conditions, water levels and the position of sewers and drains all impact on costs. Consideration should also be given to access for removal of spoil.
The Basement Information Centre estimates that groundworks for basement construction can amount for between 18% and 44% of overall construction costs. The centre has several useful models of cost estimates on its website.
This Beginners Guide appeared in the August 2020 issue of SelfBuild & Design magazine