Basement conversions in Mayfair
W1K · Central London · Dig-outs, tanking and habitable lower-ground space
Indicative local rate
£4,010 / m²
Area factor
Central London ×1.18
Typical programme
10–25 weeks
Postcode
W1K
Basement & Cellar Conversions in Mayfair
London ran out of land long ago, so the space under the house is the last easy square metreage in the building. A cellar conversion turns damp storage into a habitable room; a full dig-out lowers the slab to create proper ceiling height; a new-build basement excavates under the footprint or garden entirely.
Mayfair combines Georgian townhouses, purpose-built mansion blocks, converted mews properties and mixed-use buildings with commercial ground floors. A high proportion of the stock is listed or within the Mayfair conservation area, and many buildings run residential upper floors above offices or retail, which shapes fire strategy, acoustics and how services are routed.
That context shapes how we approach basement & cellar conversions here. Westminster's kerbside is heavily managed: loading bays need booking, skip permits are restricted, congestion and ULEZ charges apply, and noise-sensitive hours are enforced. Mews access can be too narrow for larger vehicles, so we plan hoisting, small-vehicle deliveries and off-site fabrication into the programme rather than improvising on site. We survey the property, confirm the consents that apply to W1K and issue a fixed price against a written scope, so the figure you sign is the figure you pay unless you change the brief.
Our indicative rate for basement & cellar conversions in Central London is around £4,010 per m², reflecting a ×1.18 area factor against our London baseline. Use the instant quotation tool for a figure tailored to your size, specification and start date.
Included in Mayfair
- Structural and ground investigation, plus a waterproofing design
- Planning application where required and full building control package
- Party wall notices, awards and neighbour condition surveys
- Sequenced underpinning and temporary propping
- Excavation, muck-away and new reinforced slab
- Type A, B and C waterproofing systems with cavity drain membrane
- Sump pumps, dual-pump systems with battery backup and alarms
- Drainage, ventilation, MVHR and dehumidification
How the job runs
Investigation
Trial pits, ground and water table assessment, structural survey and neighbour condition surveys.
Design
Structural design, temporary works, waterproofing strategy to BS 8102 and the fire and escape plan.
Consents
Planning where required, building control, party wall awards and a construction logistics plan.
Underpin and excavate
Sequenced pin-by-pin underpinning with monitoring, then excavation and muck-away in controlled stages.
Structure and waterproofing
New slab, retaining structure, membranes, drainage channels, sumps and pumps commissioned and tested.
Fit-out
Services, insulation, plaster, joinery, flooring and decoration to the same standard as the rest of the house.
Cellar conversion versus full dig-out
If you already have a cellar with 2.2m or more of head height, the project is largely a waterproofing and fit-out job: tank the walls, install a cavity drain system and sump, add drainage, ventilation, light and finishes. It is comparatively quick and comparatively affordable.
If head height is 1.8m, the floor has to come down — and lowering the floor means underpinning the existing walls to a new depth in a carefully sequenced order, with propping and monitoring throughout. That is a different scale of engineering, cost and programme, and it is where the majority of a basement budget goes.
Basement conversions examples in Central London
Illustrative concept case studies — not records of specific client contracts.
Basement conversions in Mayfair: common questions
How long does a basement conversion take?
A cellar conversion with waterproofing and fit-out runs 10–16 weeks. A full dig-out with underpinning is typically 6–9 months including design and consents.
Do I need planning permission?
Converting an existing cellar with no external change often does not need planning. Excavating a new basement, adding lightwells or altering the front elevation almost always does, and many London boroughs apply specific basement policies.
Will it be damp?
Not if it is designed properly. A combined waterproofing strategy with cavity drain membrane, sump pumps with backup, and controlled ventilation and dehumidification keeps a basement dry and habitable.
Is my neighbour's house at risk?
Underpinning is sequenced, propped and monitored precisely to manage that risk. Party wall awards, pre-condition surveys and movement monitoring protect both properties and are non-negotiable parts of the process.