Westminster allows a single basement storey and no more

Where in Westminster does a basement extension actually get determined?

Every Westminster postcode district we draw in, the stock that dominates it and the authority that validates a basement extension there. Westminster fees below are the statutory England rates for 2026, not basement extension estimates.

What changes about a basement extension in each part of Westminster?

Roof pitch, wall build-up and plot depth change every few Westminster streets, and each one changes what a basement extension can achieve at that address.

• Maida Vale (W9) — a basement extension in Maida Vale starts from edwardian mansion blocks, victorian villas, and Maida Vale reads as Edwardian mansion blocks, canal-side. Victorian cellars here convert without planning permission when the footprint and external appearance are unchanged in W9. Westminster City Council assesses Maida Vale basement extensions against its own Maida Vale character guidance, and the location plan we draw for Maida Vale references Little Venice.
• Mayfair (W1J, W1K) — basement extension work in Mayfair means georgian townhouses, listed buildings. Mayfair is Georgian listed townhouses, luxury. Georgian houses here already have a vault and a lower-ground floor throughout W1J. Westminster City Council decides the Mayfair application, and Berkeley Square anchors the Mayfair location plan.
• Belgravia (SW1X, SW1W) — a basement extension in Belgravia starts from regency stucco terraces, listed buildings, and Belgravia reads as white stucco Regency terraces. Victorian cellars here convert without planning permission when the footprint and external appearance are unchanged in SW1X. Westminster City Council assesses Belgravia basement extensions against its own Belgravia character guidance, and the location plan we draw for Belgravia references Eaton Square.
• Marylebone (W1U, W1G, W1H) — Georgian and Victorian terraces, mansion blocks defines Marylebone, where georgian terraces, edwardian mansion blocks set what a basement extension can do. Georgian houses here already have a vault and a lower-ground floor across W1U. Any Marylebone basement extension is validated by Westminster City Council, with Marylebone High Street the usual Marylebone reference point on the site plan.
• Pimlico (SW1V) — a basement extension in Pimlico starts from regency terraces, post-war estates, and Pimlico reads as Regency stucco terraces, grid plan. Post-war slab foundations make basements here expensive and rarely viable in SW1V. Westminster City Council assesses Pimlico basement extensions against its own Pimlico character guidance, and the location plan we draw for Pimlico references Tate Britain.
• Soho (W1D, W1F) — Georgian fragments, mixed-use defines Soho, where georgian terraces, mixed-use set what a basement extension can do. Georgian houses here already have a vault and a lower-ground floor across W1D. Any Soho basement extension is validated by Westminster City Council, with Soho Square the usual Soho reference point on the site plan.
• Covent Garden (WC2E, WC2H) — Georgian piazza, listed buildings defines Covent Garden, where georgian terraces, conversions set what a basement extension can do. Georgian houses here already have a vault and a lower-ground floor across WC2E. Any Covent Garden basement extension is validated by Westminster City Council, with Royal Opera House the usual Covent Garden reference point on the site plan.

What local planning policy applies to a basement extension in Westminster?

Westminster conservation designations: there are 56 across Westminster City Council, Soho, Covent Garden, St John's Wood among them. Designation costs a Westminster owner the permitted-development route: a basement extension then needs a £548 householder application over 8 weeks, in place of the £249 prior approval route that takes 6 weeks. Pimlico is the Westminster designation that most often changes a basement extension design.

Westminster and Article 4: Multiple Article 4 Directions — office-to-residential restrictions, HMO conversions, retail to residential in shopping zones. A basement extension planned at a NW8 address is checked against that direction first; Westminster City Council publishes the Westminster extent, and the NW8 line rarely follows the Westminster boundary you would expect.

Westminster below-ground policy: Westminster basement policy restricts to single-storey, typically under 50% of rear garden footprint; lightwells and rooflights strictly controlled in conservation areas. Any basement extension in Westminster that goes below existing foundations triggers two months notice under section 6 of the Party Wall etc. Act 1996, and Westminster City Council will want the NW8 ground and groundwater position set out.

Westminster housing stock: Grand Georgian and Regency terraces in Belgravia and Pimlico; Victorian and Edwardian mansion blocks; heritage-rich townhouses throughout; very high listed building density. Heritage-first planning approach; listed buildings on almost every street; basement and rear extension policies particularly strict. Read that before choosing a basement extension route in Westminster — construction, not postcode, sets what is achievable.

Who decides a basement extension on the edge of Westminster?

• Wandsworth — adjoining Westminster, with 45 conservation areas centred on SW8. A basement extension on the far side of the Westminster line is determined by Wandsworth Council, so the Westminster quote for basement extensions is checked against Wandsworth guidance before it is issued.
• Lambeth — the Lambeth boundary runs against Westminster, so a basement extension a street from Westminster is decided by Lambeth Council instead. Lambeth runs 64 conservation areas across SE1 and its neighbours, which is why basement extensions priced for Westminster are re-checked against Lambeth policy whenever the Westminster address sits on that edge.
• City of London — adjoining Westminster, with 27 conservation areas centred on EC1. A basement extension on the far side of the Westminster line is determined by City of London Corporation, so the Westminster quote for basement extensions is checked against City of London guidance before it is issued.
• Camden — the Camden boundary runs against Westminster, so a basement extension a street from Westminster is decided by Camden Council instead. Camden runs 40 conservation areas across NW1 and its neighbours, which is why basement extensions priced for Westminster are re-checked against Camden policy whenever the Westminster address sits on that edge.

Sources for this Westminster basement extension page: borough policies maps and conservation-area registers (2025–26), Royal Mail postcode district boundaries, the Town and Country Planning (Fees for Applications) (England) Regulations, the Town and Country Planning (General Permitted Development) (England) Order 2015 as amended, and the Party Wall etc. Act 1996.