Basement extensions: an architect’s advice

Estimated reading time: 15 minutes

Author: Becky Wootton, Architect BArch (Hons) Dip Arch

ARB registration no. 085383F

Becky is an associate at Architecture for London and she also leads our construction teams. She has direct oversight of site progress, ensuring design quality and low-energy strategies are carried through to each completed home.

Contact me: 020 3951 9790

Introduction

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

Basement spaces can be well-suited for informal family and entertaining rooms, such as a cinema, playroom, or a guest bedroom. Basements also work well for wellness spaces, including swimming pools, saunas and gyms.

View our recent basement planning approvals in London to see our previous basement projects.

Functional spaces can be accommodated in a new basement too, including plant rooms, bathrooms and utility rooms, where lower levels of natural light are acceptable.

Is a basement right for your property?

There are a series of key initial considerations:

• Estimate the cost of the basement extension with your architect. A finished basement typically costs between £6,000 and £9,000 per 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 tree 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 basement retaining walls can be up to 700mm thick, so these may reduce the usable floor space on narrow properties.

Appointing a basement extension architect

When selecting a basement extension architect, look for a practice that has experience working on similar projects in London. They will be familiar with the planning policy and the technical requirements of basements, which are often complex. An architect will guide you through the complete process: assessing your budget, developing a design that suits your needs, achieving planning consents, selecting a structural engineer, finding a builder and administering the building contract during construction.

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. Contact us for more guidance on your particular situation.

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Architecture for London is an RIBA Chartered Practice. We have current basement projects throughout London, including in Islington, Camden, Westminster and Chelsea. Contact us now on 020 3637 4236 for a 30-minute telephone consultation to discuss your project.

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In theory, achieving planning permission with a basement planning application should be relatively straightforward, as the alterations are largely below ground level and therefore less visible than other types of extension. See a typical set of planning permission outline drawings for our St John’s Wood basement.

Specific criteria for basement planning consents

The most visible part of a basement extension, and therefore the most scrutinised at the planning application stage, is often the lightwells. These provide the basement rooms with natural light and allow ventilation.

Light wells tend to be supported by local planning authorities at discreet locations, for example, at the rear of the house. Any light wells to the front of the property will be subject to design guidance and are usually encouraged to be modest in scale.

New railings surrounding proposed front light wells may be resisted in a conservation area due to their visual impact. Planning authorities may instead require glass paving or metal grilles flush with the ground level. Planting and landscaping can be designed creatively to reduce the visual impact of front light wells when viewed from the street.

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.

The planning application process should take the standard 8 weeks from submission to reach a decision. A basement can usually be submitted as a householder application, which, as of 2016, currently costs £528. Please see our article on planning application costs for more information. It is also important to create a CPP, or construction phase plan.

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.

Basement extension architects: 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, particularly toward the garden or areas of planting.
• If external views of greenery are not possible, then consider incorporating indoor plants to bring nature inside.

Light

• Admit natural daylight wherever possible, via roof lights, light pipes, light wells or courtyards that direct daylight into subterranean spaces. Try to make any light well windows full height 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 or roof lights can be chamfered to increase daylight admission.
  
  

• 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 where possible, with opening windows.
• Where mechanical ventilation is required for fresh air quality, ensure that a quiet ventilation system, also with heat recovery, 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 the basement works.
  
  

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’. All waterproofing should come with insurance-backed warranties from the product supplier and installer.

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.

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. This is often acceptable in planning terms, as the further pool excavation will be inset from the external walls. Pools are not usually considered as a ‘habitable room’ by the local authority, so daylight requirements for these spaces can be relaxed.

Basement extension architects: design and use

A basement extension can create rooms with unique qualities. In a basement swimming pool, for example, the wall might be lined with stone to enhance an architectural idea of a ‘cave-like’ space being excavated from the ground. Also, top-lit spaces can be celebrated in the design; an architect might consider the way the light from roof lights above washes down a wall surface, highlighting the texture in a wall finish.

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

Basements are also a good place to locate a plant room for M&E equipment, to free up space elsewhere in the house.

Sustainability

Ensure that your basement extension architect has a detailed sustainability strategy for the design. Basement developments produce significantly more embodied carbon emissions than similar above-ground extensions due to their use of CO2-intensive materials such as concrete and steel.

A good structural engineer will pay attention to minimising embodied energy in the basement’s structure. They will consider streamlining the use of steel and concrete, both in the temporary works and in the permanent structure. The concrete mix can also be adjusted to reduce the cement content. 

The embodied energy cost of basements can be partially mitigated over time by reduced heat loss in the house. This is achieved through careful design detailing that reduces cold-bridging. High-performance insulation and airtightness membranes also need to be specified. If the rest of the house can be upgraded with a sustainable retrofit, this can significantly reduce the overall energy requirements for space heating and, therefore, over time, help to mitigate at least some of the total CO2 impact of the basement works.

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

How much value can be added?

A basement extension in London can add significant value to your home, whilst also making it more enjoyable for you and your family. Basements typically add between £8,000 and £15,000 per square metre in London.

Basements in some areas of west London, including Kensington & Chelsea, Camden and Westminster, achieve much more than this, even up to £20,000 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.

Basement Planning applications in Westminster and RBKC

Typically, Westminster and RBKC have the most stringent policies, as this is where major disruption from ‘super basements’ has historically been more prevalent. 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 are permitted in Kensington and Chelsea since 2014.
• The depth of the slab floor to the ceiling is less of a planning concern.
• Maximum 50% of the garden rule applies.
• Appropriate planter depth of 1m for drainage attenuation.
• Construction Traffic Management Plans (CTMP’s) are a prerequisite for planning submission.
• Structural Engineering (by UK qualified engineer) design is a prerequisite for planning submission.
• Requirement for contractors to sign up to the Considerate Construction Scheme as a condition of planning consent.
• An acoustic assessment is often required.

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If you are seeking basement architects, please contact us on 020 3637 4236, or visit Architecture for London’s portfolio.

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Planning application drawing for our basement in Springfield Road, St John’s Wood, NW8

Basement extension architects FAQ