A basement conversion unlocks an entire floor of living space beneath your existing home. Whether you are converting an existing cellar, lowering the floor level, or excavating a full new basement beneath a garden, RCB Design & Build delivers basement projects across London, Kent, Surrey, and Essex. Basement work is among the most complex residential construction — it requires structural underpinning, waterproof tanking systems, specialist drainage, mechanical ventilation, and compliance with Building Regulations and the Party Wall Act 1996. As principal contractor under CDM 2015, we manage every stage from structural engineering and planning through to final fit-out. Second-fix materials (flooring, tiles, kitchen units, sanitaryware) are client-supplied so you maintain full control over the interior specification.
Convert the entire under-house area into habitable space — bedrooms, cinema room, gym, or home office. Includes underpinning, tanking, and full fit-out.
Convert an existing cellar with adequate head height into a usable room. May require floor lowering, damp proofing, and improved ventilation.
Excavate a light well at the front or rear of the basement to bring in natural daylight and meet Building Regulations for habitable rooms.
Excavate beneath the garden or beyond the existing footprint to create additional basement space. Requires significant structural engineering.
A new basement constructed beneath the rear garden, often with a light well or walk-out access to a sunken patio. Ideal for cinema rooms, gyms, or studios.
A straightforward cellar conversion with adequate head height starts from around £50,000. A full dig-out basement conversion with underpinning typically costs £80,000–£150,000 depending on the size of the area, depth of excavation, ground conditions, and specification. We provide a fixed written quotation after a site visit and structural assessment.
Converting an existing cellar within the existing footprint does not usually require planning permission, though Building Regulations approval is always needed. New basement extensions (dig-outs beneath gardens or beyond the existing footprint) typically require full planning permission. Many London boroughs have specific basement policies. We advise on the planning route during the initial consultation.
We use a Type C cavity drain membrane (CDM) system, which is the most reliable method for below-ground waterproofing. This involves fitting a studded membrane to the walls and floor, directing any water ingress to a sump pump that discharges to the surface drainage. The system is maintainable and comes with a long-term guarantee.
If you are underpinning or excavating within 3 metres of a neighbouring property (6 metres if excavating below their foundation line), you must serve a party wall notice under the Party Wall Act 1996. We manage the entire party wall process on your behalf, including appointing surveyors and agreeing the award.
A cellar conversion with no underpinning takes 12–14 weeks. A full dig-out basement with underpinning takes 16–24 weeks depending on the size and complexity. The underpinning phase alone can take 4–8 weeks as it is carried out in carefully sequenced sections. We provide a detailed construction programme at contract stage.
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