Flat Conversions Westminster

Flat conversion work in Westminster starts from a stock that is 91% flats and 9% houses, so the houses left to divide are few. The jobs are splitting a pre-1919 townhouse into self-contained flats, putting right an older conversion, turning a commercial floor into homes where the rules allow it, and occasionally joining flats back into one house. Each route brings its own planning, space, fire and sound requirements.

In Westminster, converting a house into two or three flats is published at £50,000 to £120,000 including professional and council fees, and an England-wide guide puts a straightforward two-flat split at £40,000 to £80,000 in construction alone, which is the part a builder prices (indicative London ranges, Architectural Drawings and Mayfair Studio, 2026). GS Renovation Ltd, a building contractor based in Potters Bar, does the work in Westminster: we build the conversion to the approved drawings, building control inspects the fire and sound separation, and a registered electrician certifies each flat's electrics.

Which Westminster buildings are converted into flats

The candidates in Westminster are a small group of buildings. Terraced houses make up 8% of the City's homes and semi-detached or detached houses 1%, against 48% of all homes built before 1919. That points to tall period townhouses and the upper floors of commercial buildings rather than suburban semis. Splitting a house into flats is always a material change of use, so the owner or their designer obtains planning permission before any wall moves. For addresses in the Oxford Street Development Corporation area, planning decisions now sit with the OSDC rather than Westminster City Council.

The London Plan counts residential conversions among the small sites that add housing, but only where they result in net additional homes. Westminster charges CIL per square metre on new dwellings of any size, but it bites on new floor space. A conversion inside a house in lawful use usually pays little or nothing unless it adds area, so the owner's designer or planning agent confirms the liability early. Where the house is listed, the conversion also needs listed building consent, with around 11,000 buildings and structures on the list in Westminster.

Housing in Westminster

For Westminster the Council Tax list gives about 132,230 homes; by type 91% flats and 9% houses, by age 40% post-war, 10% interwar and 48% older than 1919. Since most homes here are flats, a flat in Westminster can be split or combined only with the freeholder's consent, and changing the number of homes usually needs planning permission as well.

Room sizes, ceiling heights and aspect for each new Westminster flat

A new flat carved out of a Westminster house must reach the London Plan minimums, whether it is for sale or for rent. Policy D6 counts a conversion or change of use as a new dwelling, so the same table governs a split townhouse as a new block. A single bedroom needs 7.5 square metres of floor and 2.15m of width, and a double or twin needs 11.5 square metres.

Height matters as much as area in this stock. Policy D6 wants 2.5m floor to ceiling across at least 75 per cent of each dwelling's internal area, which a tall piano nobile passes easily and a low attic storey often does not. D6 also asks schemes to maximise dual aspect homes and normally avoid single aspect ones. We check the measured survey against these tests before the plan is fixed, so partitions are set out once.

Fire, sound and services between flats in a Westminster conversion

Separating the flats for fire and sound is the main building regulations work in a Westminster conversion. The ratings for separating floors and the protected stair come from Approved Document B and the approved design, and a taller building needs more than a two storey one. Because the layout changes, full plans is the route where Westminster is the building control body. Where new steel or a separating wall bears on a party wall, the work needs a party wall notice, which the building owner serves. We build to the ratings building control approves and leave the floors open for inspection.

For sound, Approved Document E sets targets for flats formed by a change of use: 43 dB airborne through separating walls, floors and stairs, and no more than 64 dB impact through floors and stairs. On timber joists that usually means a new ceiling hung on resilient bars, mineral wool between the joists and an acoustic layer over the boards, finished before pre-completion testing. Each flat then gets its own meters, supplies and a postal address through street naming and numbering.

Turning a Westminster office or shop floor into flats

Office or shop to flats under permitted development is closed in named parts of Westminster. The Central Activities Zone direction, made in July 2021 and modified in 2022, still reaches the West End and Knightsbridge International Centres, and a second Class MA direction, confirmed on 5 December 2022, covers the town centre hierarchy beyond it. Outside those areas, the owner's planning route still needs confirming before we price the strip out.

Homes created under Class MA must meet the nationally described space standard, a condition written into the permitted development order itself. Since 5 March 2024 Class MA has no floor space cap and no vacancy condition. The building must have been in Class E use for the two years before the application, prior approval is needed, and the right does not apply to a listed building. Deep commercial plans also need natural light into every habitable room, which prior approval tests, so light wells and new windows are drawn in early.

• Fire stopping round every pipe and cable through a separating floor
• A kitchen and shower room fitted in every new flat
• Separating walls built and lined for sound between flats

VOA Council Tax stock of properties, March 2025, for all 24 statistical areas in Westminster. Counts rounded by the VOA.

Alongside flat conversions in Westminster, the pages that usually help next are structural alterations in Westminster, our Westminster area guide, a building contractor for the whole conversion, party walls in a flat conversion, flat conversions anywhere in London and Party Wall Act Explained.

Why Westminster Homeowners Choose GS Renovation

A Westminster conversion is a sequence of fire, sound and services work that has to be right before the finishes go on. We build to the approved drawings, coordinate the owner's structural engineer, the registered electrician and a registered gas engineer, and set the order so fire separation and acoustic floors are inspected before ceilings close. You get a written programme for each flat, separate supplies ready for the meters, and the certificates building control needs at the end.

• Mains-wired, interlinked smoke and heat alarms to the approved fire strategy
• Every flat checked against the space standard before the layout is priced
• Rated 4.9 from 31 reviews on MyBuilder

Checked by Abdelaaziz Saaid, founder and director of GS Renovation Ltd (Companies House no. 14611254). Structural design is by the consulting engineer appointed to your project.