Home Extension Design in Westminster: Planning, Costs and What's Realistic in 2026
A home extension in Westminster typically costs £4,000 to £5,500 or more per m² to build in 2026, at mid-range specification, before VAT and professional fees. Heritage adds to that: conservation area work carries roughly 15% on top and listed buildings 30% or more. Assume you will need planning permission. Westminster has 56 conservation areas covering most of the city, and a city-wide Article 4 Direction has required planning permission for every basement since 2016. This page covers what the council allows and what it genuinely costs.
Studio MASS is a residential design practice led by an architectural technologist. We design and deliver extensions, refurbishments and heritage-sensitive alterations across London, from first feasibility check to completed build.
Do you need planning permission for an extension in Westminster?
Almost certainly, yes. Westminster has 56 conservation areas, and inside them planning permission is needed for demolition, for side extensions, for rear extensions beyond 3 metres and for any rear extension of more than one storey, along with some alterations that would not need permission elsewhere. Listed buildings, which Westminster has in unusual concentration, need listed building consent for internal as well as external works. The safe working assumption for a Westminster home is that something will need consent.
Several conservation areas carry additional Article 4 Directions removing permitted development rights for minor external alterations: Abbey Gardens, Bridstow Place, Bristol Gardens, Moncorvo Close, the Queen's Park Estate, Relton Mews and Sussex Gardens. On the Queen's Park Estate, for example, changes to windows, doors and other visible details need an application that would be permitted development a borough away.
None of this makes extending impossible. It makes the order of work matter: check the property's designations first, understand what the conservation area audit says about your street, and design to that context. That check is part of our Stage Zero review, before any design work starts.
Can you still build a basement in Westminster?
Yes, but never under permitted development. A city-wide Article 4 Direction, in force since 31 July 2016, removes permitted development rights for basement development throughout Westminster, so every basement needs a planning application. City Plan policy then shapes what gets approved: basements are generally limited to a single storey and must leave a proportion of garden land undeveloped, with structural and construction-management evidence expected up front.
Cost is the other filter. Basement space runs £6,000 to £8,000 or more per m² under the garden and £8,000 to £12,000 or more per m² under the house, before the heritage uplifts that apply to most Westminster properties. For many homes a well-designed rear or roof-level alteration delivers more value per pound.
How much does a home extension cost in Westminster in 2026?
A single-storey extension in Westminster costs £4,000 to £5,500 or more per m² at mid specification in 2026, reflecting the borough's inner-London zone 1 and 2 location, restricted site access and heritage detailing. Conservation area projects carry roughly 15% on top, and listed buildings 30% or more. A loft conversion starts at £3,200 to £3,800 per m² at mid specification before those uplifts. These figures are construction only; VAT at 20% and professional fees are additional.
The housing stock explains the premium. Stucco terraces in Pimlico and Bayswater, Georgian townhouses in Marylebone, villas and mansion flats around Maida Vale and Little Venice, and the tight Victorian cottages of the Queen's Park Estate all reward careful, period-appropriate work and punish shortcuts. Mews houses bring their own access constraints, with materials often carried in by hand.
Before a brick is laid, expect £8,000 to £16,000 in professional and statutory fees on a typical project, including the £548 planning application fee that applies from April 2026, and more where listed building consent and heritage statements are involved. The full breakdown by extension type and specification level is in our free Extension Cost Guide 2026, no email required.
Who designs your extension if not an architect?
An architectural technologist designs buildings with a focus on how they are built: construction detail, building regulations, and buildability, alongside layout and light. In the UK "architect" is a legally protected title, so we don't use it. What matters for a Westminster extension is the same either way: design within the local planning context, technical drawings a builder can price accurately, and someone accountable from first idea to finished build. That is what we do, through one team and five stages, starting with a Stage Zero review that tells you what is realistic for your budget and what planning will allow before design begins.
Start with a conversation
The best starting point is our short questionnaire. It takes a few minutes and tells us about your home, your brief and your budget. We review it and arrange a free thirty-minute call, with an honest view on feasibility either way.
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