Basement extensions in City of London

City of London does not have a borough-specific basement policy. National framework applies — structural, drainage and flood-risk assessments required.

Typically single-storey under 50% of garden footprint, with light wells and rooflights subject to streetscape control. Double-storey basements are restricted or refused in most central London boroughs (Westminster, RBKC, Camden).

Construction (separate from drawings): £4,500–£6,500 per m² for a single-storey rear basement under garden. Lightwell, drainage and underpinning drive cost. Drawings: from £4,500 fixed fee for the full planning + structural package.

Structural engineer report, ground conditions report, hydrology/drainage assessment, Party Wall notices, and Construction Management Plan. Conservation areas add Heritage Statement.

Almost always yes. Excavation within 3m of a neighbour's foundation or within 6m where the basement is below their footing level triggers the Party Wall Act. Surveyor fees typically £1,000–£3,000 per neighbour.