Expert basement conversion builders across London. Post a job free — specialists contact you within hours.
Transform an existing cellar into a habitable room — cinema room, gym, home office, or guest bedroom. Waterproofing and structural work included.
Get quotes →Excavate a new basement where none exists. Complex structural engineering and waterproofing, but adds significant value in high-price London areas.
Get quotes →Extend the basement footprint beyond the existing property footprint. Usually requires planning permission. Specialist structural engineers required.
Get quotes →Grade 1, 2 or 3 waterproofing systems installed to BS 8102 standards. Cavity drain membranes, tanking renders, and sump pump systems.
Get quotes →Party wall notices served and awards agreed with neighbouring properties — essential for all London basement projects.
Get quotes →Excavated light wells bring natural light into below-ground spaces. Grilles, drainage, and safety barriers included.
Get quotes →| Job | Low estimate | High estimate |
|---|---|---|
| Basement conversion (existing cellar) | £40,000 | £80,000 |
| New dig-out basement | £80,000 | £150,000 |
| Structural engineer fees | £2,000 | £5,000 |
| Waterproofing / tanking | £5,000 | £15,000 |
| Party wall surveyor (per neighbour) | £700 | £2,000 |
| Light well installation | £3,000 | £8,000 |
| Planning application fee | £206 | £206 |
| Building regulations approval | £800 | £2,000 |
Prices are estimates for London 2026. Basement costs vary significantly by ground conditions, access, and waterproofing grade. Always get multiple quotes.
Describe the basement project (existing cellar or new dig-out, intended use, approximate size) and your London postcode.
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Basement conversions are one of the most ambitious — and potentially most rewarding — home improvement projects in London. In boroughs where property values are exceptionally high, such as Kensington, Chelsea, Islington, and Westminster, a well-executed basement adds more space without encroaching on the garden or changing the external footprint of the property. Cinema rooms, gyms, wine cellars, home offices, and guest suites are the most common uses. For properties with existing cellars, conversion is significantly less expensive than a new dig-out, as the excavation and structural underpinning work is already largely done. Waterproofing to the correct grade for the intended use is critical — London's clay soils and London Underground tunnels in many areas mean groundwater management is non-trivial.
New dig-out basements — excavating a full basement where none previously existed — are complex structural projects requiring specialist engineers and contractors. Party wall agreements with all affected neighbours are almost always required, and the process can take 2–3 months from notice to award. Some London boroughs have introduced planning policies restricting basement conversions to a single storey below ground, limiting the depth and preventing the construction of swimming pools or leisure facilities that have proven controversial in dense neighbourhoods. Always engage a planning consultant or experienced basement builder to assess the feasibility of your project before commissioning structural drawings.
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