Basement extensions: an architect’s advice in 2023

This article highlights the key considerations when undertaking a basement extension project, from appointing an architect to designing the spaces and gaining planning consents.

A basement extension can create new rooms with unique qualities. From an architectural perspective, subterranean spaces may not be perfect for a kitchen or dining room due to reduced natural light and views.

Architecture for London has a track record of successful consents for basement projects throughout London, including in Islington, Camden, Haringey, Westminster and Kensington & Chelsea. View our portfolio of recent work, or contact us on 020 3637 4236 to discuss your project.

Is a basement right for your property?

There are a series of key initial considerations:

• Estimate the construction costs with your architect. A finished basement typically costs between £4,500 and £6,000/sqm, excluding VAT and fees.
• Does the potential value added to the house cover the construction cost? Show your plans to local estate agents to estimate the value added.
• Consider a desktop study of your property: what lies on or beneath your site? Are there watercourses nearby? Does the area have a history of flooding?
• There are additional controls on trees works in conservation areas and root protection zones need to be considered. An arboriculturist’s report can make a case for the removal or replacement of a tree.
• Consider that retaining walls can be up to 700mm thick, so these may reduce the useable basement floor space on narrow properties.

Architecture for London created a basement extension to a house in Islington. Oak veneered joinery conceals a pull-down bed that allows use as a flexible guest bedroom.

Appointing an Architect

When choosing an architect, look for a practice that has experience of working on basement extensions, as they will be familiar with local planning policy and the technical requirements of these projects, which are often complex.

Achieving planning permission for a basement

Planning permission may not be required in some instances, for example when extending an existing basement or cellar in a house that benefits from permitted development rights. In most other situations, however, you will need planning permission for a basement extension in London. Speak to an architect for more guidance on your particular situation. 

Specific criteria for planning consent

The most visible part of a basement extension, and therefore the most scrutinised at planning application stage, is usually the lightwell. This provides the basement rooms with natural light and allows ventilation.

Lightwells tend to be supported by local planning authorities at discreet locations, for example at the rear of the house. Any lightwells to the front of the property will be subject to design rules and are encouraged to be modest in scale.

Railings to front lightwells tend to be resisted, in favour of glass paving or grilles flush with the ground level. Landscaping can be designed creatively to reduce the impact of front lightwells when viewed from the street.

A basement extension in Westminster with a glazed wall to the rear lightwell to maximise light.

In London, there are usually also restrictions on the total size of the basement created. Many London boroughs are now using standard criteria, creating more policy consistency throughout the city. The most common criteria for basement sizes in the borough’s SPD (supplementary planning documents) are as follows:

• Basements must not exceed the property footprint, plus a maximum of 50% of the garden.
• They must not be more than one storey.
• Further basement floors will not be approved where there is an existing implemented permission or one built through permitted development rights.

How long does it take?

The outline design phase is likely to be extended due to the additional technical input required from other consultants at this stage, particularly basement impact assessments, construction methodology statements and calculations from a structural engineer.

A basement can usually be submitted as a householder application, which currently costs £206. Please see our article on planning application costs for more information.

The detailed design and construction stages are likely to take longer than an above ground extension, due to the extra time required to design and carry out excavation, underpinning of the existing house, waterproofing etc.

AFL achieved planning consent for a basement extension with a front lightwell in Islington

Detailed design considerations

The main factors to consider when designing a basement are as follows:

Views

• Try to establish visual connections with the outside wherever possible.
• Consider how to incorporate outdoor and indoor planting to improve your enjoyment of the space.

Light

• Admit natural daylight wherever possible, via roof lights, light pipes, lightwells or courtyards that direct ambient daylight into subterranean spaces. Try to make the lightwell windows full height if possible by dropping the cills, to take full advantage of the light available.
• In sensitive locations, consider creative approaches to concealing roof lights. For example, creating fishponds or water features with glass bottoms in your garden.
• Reveals around windows can be chamfered to increase daylight admission.

A rear and basement extension to a house in Highgate, finished in brick and stone.

• Consider room plan depths in relation to the use of the space. Utility rooms or wine cellars are often located in the darker areas toward the middle of the plan.
• Increase floor to ceiling heights as much as possible to make rooms generous and bright.
• Use light reflective paints
• Use light coloured materials and finishes, particularly on floors and ceilings.
• Use artificial lighting as necessary – consider wide angle lenses and beams.
• Specify discrete ceiling lights that create soft, ambient light on walls and floors. Avoid intense washes of light.

Ventilation, heating and cooling

• Make use of natural ventilation as far as possible, with opening windows.
• Where mechanical ventilation is required for fresh air quality, ensure that low noise ventilation is specified. Mechanical and electrical consultants can advise on system technical performance.
• Bear in mind that additional space may be required to accommodate the plant for ventilation.
• Improvements to the thermal performance of the rest of the house should be carried out at the same time as basement works.

Architecture for London gained consent for a large basement, with a car lift and swimming pool, at this detached house in Christchurch Street, Chelsea.

Waterproofing

• This is usually designed to be site specific. Your architect will work with specialist waterproofing suppliers and installers who advise on system technical performance.
• On constrained sites in London, membrane systems are typically laid to the inside face of concrete retaining walls in conjunction with perimeter drainage. Either gravity fed or pumped sump drainage systems are necessary as the walls outside of the membrane remain ‘wet’. 

Drainage

• Basement works are likely to alter your property’s existing surface and foul water drainage. An appropriate strategy should be considered from the outset.
• Specialist underground drainage consultants can advise on system technical performance.
• If your basement is going to extend under your garden, enough soil depth should remain for the rootball growth of planting and to control stormwater attenuation. A rule of thumb is to leave one metre depth of soil above the slab.
• Some local authorities will have more stringent requirements for drainage design.
  

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Cellars and existing basement conversions

For buildings that are not listed, it is usually possible to excavate one metre from the existing cellar floor level without requiring any permissions. This significantly improves ceiling heights, which is often a key limitation in an existing cellar. Contact us for more details on your particular situation.

Creating a swimming pool often allows additional basement excavation depth. Local authorities tend to support this, as the further pool excavation will be inset from the external walls.

Architecture for London lowered floor levels in an existing cellar by 600mm. A kitchenette was created in a vault that previously had a ceiling height of only 1.2m.

How do you want to use your basement?

Popular uses that are well suited to basement extensions include:

• Gym, swimming pool and wellness facilities – spa, hammam, sauna and steam rooms
• Games rooms
• Media and cinema rooms
• Playrooms
• Utility rooms
• Libraries
• Music and recital rooms
• Wine display and stores
• Secure rooms for valuables
• Seasonal storage
• Vehicle storage

Sustainability

Basement developments produce more embodied and construction CO2 emissions during a typical sixty-year lifecycle than similar above-ground extensions due to the use of CO2-heavy products such as concrete.

The embodied energy can be partially mitigated over time by reduced heat loss, because basements are below ground and benefit from the natural insulation properties of the soil.

A SUDS (sustainable drainage system) strategy should be considered. A diagram will be required to indicate that any new below-ground development does not result in additional hard surfaces externally.

How much value does a basement add?

A basement extension can add significant value to your home, whilst also making it more enjoyable for you and your family. Basements typically add between £7,500 and £10,000 per square metre in London.

Basements in some areas of Kensington & Chelsea, Camden and Westminster achieve much more than this, even up to £15,000 per square metre.

Basement extensions under the garden

Current supplementary planning documents in the London boroughs generally offer support for single story basements with a depth of up to 4m under the footprint of the entire house plus up to 50% of the garden area.

There is sometimes scope for site-specific criteria to be employed to achieve a basement that is larger than 50% of the garden. This may be the case where neighbours are further away from the property. Basements under the garden area can be in the side, back or front garden.

Policy in Westminster and RBKC

Typically Westminster and RBKC have the most stringent policies, as this is where ‘super basements’ have been more prevalent, and other boroughs tend to follow suit. Some of the key policies in Westminster and Kensington & Chelsea are as follows:

Westminster

• Areas along the river in Westminster are at risk of flooding, and basements in vulnerable areas will not be permitted.
• In listed buildings, historic features like vaults in cellars, must be retained.
• A structural methodology statement will be required.
• Appropriate planter depth of 1m for drainage attenuation.
• Planning pre-commencement conditions are often extensive.

Kensington and Chelsea

• Single storey basements only, since 2014.
• Depth of slab floor to ceiling is less of a planning concern.
• Maximum 50% of garden rule applies.
• Appropriate planter depth of 1m for drainage attenuation.
• Construction Traffic Management Plans (CTMP’s) a pre-requisite of planning submission.
• Structural Engineering (by UK qualified engineer) design a pre-requisite of planning submission.
• Requirement for contractors to sign up to Considerate Construction Scheme as a condition of planning consent.
• An acoustic assessment is often required.

Diagram illustrating 50% of the garden rule, at Architecture for London’s basement extension project in Christchurch Street, Chelsea.

To see Architecture for London’s recent basement projects, please visit our project portfolio.

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