Basement conversion plans in Westminster

Nearly always. Excavating to create new space, deepening a cellar or forming a lightwell goes beyond permitted development, and most inner-London boroughs — Westminster City Council included — run a specific basement or subterranean-development policy. A full planning application is the norm, usually supported by a Basement Impact Assessment. We confirm the exact policy route for your Westminster address before any drawing starts.

A Basement Impact Assessment is a technical report on how the dig affects groundwater, drainage, structure and neighbouring buildings, and most Westminster basement schemes need one to satisfy Westminster City Council's policy. It is the document planners lean on most, because it shows the excavation has been thought through rather than assumed. We coordinate it with the structural design so the two tell the same story.

Almost certainly. A basement means excavating close to — and often underpinning beneath — a neighbour's wall, which triggers the Party Wall etc. Act 1996. You must serve notice on the adjoining owners and usually reach a party-wall award before you dig. We set the drawings up so the party-wall surveyor works from the same sections as the engineer, which keeps a Westminster basement out of dispute. Westminster City Council is the decision-maker.

A basement sits below ground and, for part of the year, below the water table, so it is designed to be watertight rather than merely damp-proofed. The usual approach is a Type C cavity-drain system that collects any water behind the walls and floor and takes it to a sump and pump. We draw the waterproofing to the current British Standard and coordinate it with the structure, because in a Westminster basement the tanking and the retaining wall have to work as one. Westminster City Council is the decision-maker.

Sometimes, but it is harder. A listed house — or one in a conservation area such as Bayswater and Maida Vale — needs listed building consent for any work affecting its fabric, and Westminster City Council scrutinises lightwells, vault alterations and the effect on the historic structure closely. It can be done where the scheme protects what matters, but the justification has to be thorough. We assess the designation honestly before recommending a dig.

We price each Westminster basement individually, because the depth, the ground conditions, the party-wall count and the planning policy all vary house to house, and send a fixed, no-obligation quote within one working day. It covers the coordinated planning and technical set with free revisions until Westminster City Council approves. The structural engineer, Basement Impact Assessment specialist and any council fee are separate. Call 020 7864 7245 to start.