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Achieving Planning Permission For A Basement

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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. In theory, gaining consent for basement applications should be relatively straightforward, as the alterations are largely below ground level and therefore less visible than other types of extension.

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.

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 currently costs £206. 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.

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


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 plant and M+E equipment, to free up space elsewhere in the house.

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 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 theborough’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 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. 

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