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ASK ARCHITECT: Is a basement extension right for my property?

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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. Basements can, however, work very well for different uses.

1001 Homes has a track record of successful consents for basement projects throughout London, including in Islington, Camden, Haringey, Westminster and Kensington & Chelsea and in other counties like Oxfordshire. View our recent work, or contact us on 07746055146 to discuss your project.

Is a basement right for your property? There are a series of key initial considerations:

Estimate the construction cost 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.

Appointing an Architect

When choosing architect for your basement extension 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. An architect will guide you through the complete process: assessing your budget, developing a design that suits your needs, achieving planning consents, selecting a builder and assisting during construction.


1001 Homes achieved planning consent for a basement extension with a front light well in Islington.

Can you extend your basement?

Consider what lies on or beneath your site: are there watercourses nearby and does the area have a history of flooding? Are there any protected trees that might be affected? If your property is very narrow, a basement may not be suitable given that retaining walls can be up to 700mm thick, reducing useable floor space.

Are basements permitted development?

For a house that benefits from permitted development rights, it may be possible to build a basement extension without planning. However, external features like railings or light wells usually require consent.

Can you put a bedroom in a basement?

Basements can contain bedrooms if they have adequate daylight and ventilation, this will be scrutinised during the planning application process. A secondary means of escape may also be required to comply with the building regulations.

How much value does a basement add?

Basements typically add between £7,500 and £10,000 per square metre in London. In some areas of Kensington & Chelsea, Camden and Westminster this figure could be up to £15,000 per square metre.

Will I get planning consent for a basement?

In London, any detailed guidance in your borough’s SPD (supplementary planning documents) should be followed to maximise your chance of gaining consent. These often limit the footprint of the basement to the area under the existing house plus no more than 50% of the area under the garden. They are also usually restricted to a single storey in depth.

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