Multi-Level Basement
in Belgravia, SW1X

A multi-level basement in Belgravia is shaped by policy before engineering. Westminster's City Plan 2019–2040, adopted in January 2026, supports basements of no more than one storey beneath the lowest original floor level, describing its preferred form as a single storey of about 2.7m floor to ceiling, with exceptions possible on large sites that can take plant and machinery and have access for construction without harming neighbours.

For properties under Grosvenor's Belgravia Estate Management Scheme, a basement also needs Grosvenor's consent, even though it does not change the external look of the house, alongside planning permission and, where relevant, listed building consent. Westminster's 2004 guide to the Belgravia Conservation Area, designated in 1968, records approximately 500 Belgravia properties listed Grade II, and the Belgravia Neighbourhood Plan records basement excavation, sometimes double-depth, among recent enlargements. Below is what each says, and how our design-and-build team approaches the engineering.

Sources: Westminster City Plan 2019-2040 (2026, adopted January 2026), Grosvenor: Belgravia and Mayfair management schemes, Westminster City Council: Belgravia conservation area mini guide, Belgravia Neighbourhood Plan 2023-2040, Adoption Version (March 2024), published by Westminster City Council

Grosvenor's alterations guide, written for freehold owners, describes three consents you may need, each with a distinct process: its own, listed building consent and planning permission. Each basement application is considered on its merits, with factors including listing, house type, engineering complexity, ground conditions, the garden and access. Its guidance says:

• a basement storey should not exceed 3.0m from slab to structural ceiling, within a maximum total excavation depth of 4.2m;
• excavation below floors already below street level is generally not permitted unless they are under 2400mm floor to ceiling, when digging to reach that height is allowed;
• excavations conflicting with a 45° line from an adjacent listed building's foundations are unlikely to be approved;
• basements are generally discouraged in mews houses.

Approval carries conditions before, during and after the works. Grosvenor may appoint structural engineers and geotechnical and acoustic consultants to review and approve the detailed design, may require a premium where a scheme adds floor area, and in Belgravia also controls changes to internal load-bearing walls.

We take initial sections to Grosvenor's Building Surveying team before applying for planning, so the estate's depth and 45° limits are built into the scheme Westminster assesses.

Source: Grosvenor: Belgravia and Mayfair management schemes

The Belgravia Neighbourhood Plan records basements flooding in heavy rain in July 2021, especially in Pimlico Road, Bourne Street and Chester Row, where, it says, the water table is very near the surface, close to the submerged River Westbourne. Grosvenor warns that excavating through the water table brings construction challenges and uncertainty about water diversion, and that a rigid concrete basement beside shallow lime-mortar foundations can cause excessive movement and cracking in neighbouring buildings. Westminster requires a structural methodology statement for every basement, a Flood Risk Assessment in flood zones 2 and 3 or surface water hotspots, and measures such as sustainable drainage and positive pumped devices or equivalent.

Our engineers start with boreholes and the measured groundwater level, then design the retaining walls, temporary propping and underpinning sequence. Waterproofing is layered, typically a concrete box with a drained cavity membrane and a pumped sump with a standby pump. Where a second level is in question, the ground investigation decides it.

Sources: Belgravia Neighbourhood Plan 2023-2040, Adoption Version (March 2024), published by Westminster City Council, Grosvenor: Belgravia and Mayfair management schemes, Westminster City Plan 2019-2040 (2026, adopted January 2026)

The Neighbourhood Plan notes that period houses in terraces depend on the whole terrace for their stability, and that major works, especially excavation, can affect up to two neighbouring buildings either side. Section 6 of the Party Wall etc. Act 1996 covers excavation within three metres of a neighbour's building to below its foundations, or within six metres where a 45-degree line is crossed; the notice comes with plans and sections of the dig, and the owner may, and if required must, underpin or otherwise safeguard the neighbour's foundations, so far as necessary, at their own expense. Westminster strongly encourages consulting neighbouring occupiers before applying.

We commission schedules of condition next door and agree monitoring points and trigger levels with the neighbours' surveyors.

Sources: Belgravia Neighbourhood Plan 2023-2040, Adoption Version (March 2024), published by Westminster City Council, legislation.gov.uk - Party Wall etc. Act 1996, section 6, Westminster City Plan 2019-2040 (2026, adopted January 2026)