How much does a basement conversion cost in London?
Our fixed fee for the drawings runs from £1,995 to £2,895, from converting an existing cellar to a new-build basement or light well. The construction bill is a separate and much larger figure that depends on depth, ground and structure. We hold no sourced build price, so we quote none.
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What our drawing fee covers
A cellar conversion drawn for building regulations starts at £1,995 on our published price list, with the structural engineer priced separately. Our fixed fee range of £1,995 to £2,895 covers work from converting an existing cellar to a new-build basement or light well.
Those fees buy the drawings you need to price the job and to apply for consent. They do not include digging, underpinning or finishes, which a builder prices from your drawings.
Why the build figure varies so widely
Converting an existing cellar works within a space that is already there. A new basement means excavating under the house, supporting the structure while it is done, and managing water in the ground. The two are different jobs, which is why we price the drawings separately for each.
Ask for structural engineering input early. Our structural drawings service and the guide to structural calculations explain what the engineer provides.
A householder planning application for enlarging a single home costs £548 from 1 April 2026, where a basement needs one.
Digging near a neighbour: the Party Wall etc. Act 1996
Section 6 of the Act applies to excavation close to a neighbouring building. It covers digging within three metres, measured horizontally, to below the neighbour's foundations. It also covers digging within six metres where the excavation meets a plane drawn down at 45 degrees from the neighbour's foundations (Party Wall etc. Act 1996, section 6).
Notice of the excavation must be served at least one month before you begin to dig. Surveyor fees come on top of the drawings and the build, so allow for them in the budget. Our party wall agreement guide explains the process.
Application fees and the building regulations clock
A householder planning application for enlarging a single home costs £548 from 1 April 2026, where a basement needs one. Whether it does depends on the design, and the basement planning permission guide covers when it does.
For the building regulations, a full plans application gets its decision within five weeks of receipt, or a longer period you agree (Building Regulations 2010, regulation 14A).
Pricing the dig and the build
Give each contractor the same drawings and structural design, then compare quotes line by line. The basement conversion drawings page lists what our fixed fee includes.
Sources
Every figure on this page, where it comes from and a link to the original.
| Figure | Source | Link |
|---|---|---|
| £1,995–£2,895 | DrawingsDirect fixed fee range from converting an existing cellar to a new-build basement or light well | drawingsdirect.co.uk |
| £1,995 | DrawingsDirect starting fixed fee for basement conversion of an existing cellar (building regulations; structural engineer priced separately), from the published price list | drawingsdirect.co.uk |
| three metres | Party Wall etc. Act 1996 s6(1)(a): notice applies to excavating within three metres of a neighbouring building or structure to below its foundations (source wording: "within a distance of three metres measured horizontally") | www.legislation.gov.uk |
| six metres | Party Wall etc. Act 1996 s6(2)(a): notice applies to excavating within six metres of a neighbouring building where the excavation meets a plane drawn down at 45 degrees from its foundations (source wording: "within a distance of six metres measured horizontally") | www.legislation.gov.uk |
| one month | Party Wall etc. Act 1996 s6(5): notice of an excavation is served at least one month before excavating begins (source wording: "at least one month before beginning to excavate") | www.legislation.gov.uk |
| £548 | Fees for planning applications in England from 1 April 2026 (MHCLG, SI 2012/2920 as annually indexed), category 6(1): householder application for the enlargement, improvement or alteration of a single dwellinghouse (source wording: £548) | assets.publishing.service.gov.uk |
| five weeks | Building Regulations 2010 reg 14A(6): the authority must notify the outcome of a full plans application within five weeks of receipt, or a longer agreed period (source wording: "within five weeks beginning with the date the application is received") | www.legislation.gov.uk |
Questions people ask
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How much does a basement conversion cost in London?
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Photographs: W.carter (CC0).
