Basement Conversions in London

London basement conversions typically range from around £45,000 for a simple cellar dig-out and conversion through to £180,000+ for a full underpinned basement with lightwells, en-suite and high-end finishes. Most full conversions land between £70,000 and £130,000 depending on depth of dig, underpinning requirements and finish level. We provide a written quote after the free site visit.

Allow 4 to 9 months from site start to handover for a full underpinned basement. A straight cellar conversion (no dig-down, existing head height) can be done in 8 to 12 weeks. Underpinning alone typically takes 6 to 10 weeks before main works begin. We'll give you a stage-by-stage programme with your quote so you know what's happening when.

A simple conversion of an existing cellar usually falls under permitted development and doesn't need planning permission. Digging down to create a new basement, or adding a lightwell visible from the street, almost always does — and some London boroughs (Kensington & Chelsea, Westminster, Camden) have stricter basement policies than others. Building Regulations apply to every basement conversion regardless. We'll advise on what's needed at the site visit.

If your basement work affects a shared wall or comes within 3 metres (sometimes 6 metres) of a neighbour's foundations, the Party Wall etc. Act 1996 applies. You'll need to serve formal notices and most neighbours will appoint a surveyor at your cost. We co-ordinate this from day one and have worked with most of the London party wall surveyors — it adds time at the start but avoids legal headaches mid-build.

Every basement we build uses a combined Type A (external/structural tanking) and Type C (internal cavity drain membrane) system to BS 8102, feeding into a twin-pump sump chamber with battery backup. Belt and braces. The cavity drain system is the long-term insurance — if any water ever does find a route in, it gets channelled to the sump rather than into the room.

Yes — that's the whole point of how underpinning is sequenced. It's done in short sections (usually 1 metre at a time) in a hit-and-miss pattern so the existing foundations are always supported. The structural engineer specifies the sequence and we don't move on until each section has cured. We also monitor movement during the dig with telltales on any cracks.

Most clients stay in the upper floors throughout, yes. It's noisy and dusty during underpinning and excavation (roughly the first half of the project) and quieter once we're on first fix and finishes. We seal off the basement access, set up dust screens, and agree working hours that suit you. If you want to move out for the heavy phase we'll help plan the timing.