House Extensions Westminster
Single storey rear and double storey extensions in Westminster, planning, structure and build under one contract.
- Rated 4.9 from 31 reviews on MyBuilder
- Single-storey rear extension from £40,000
- Free site survey, itemised quote
GS Renovation plans and builds extensions across Westminster, working with the grand stucco terraces, period mansion blocks and prestigious townhouses of Mayfair, Pimlico and Paddington. We handle groundworks, steel, roofing and full fit-out on properties that are often listed or within a conservation area.
In Westminster, a single-storey rear extension typically starts from £40,000, or £2,000 to £2,800 per m² (indicative London ranges, Architecture for London, Resi, MyBuilder and Checkatrade, 2026). GS Renovation Ltd, a building contractor based in Potters Bar, does the work in Westminster: the structural engineer we appoint designs the structure, we build it, and building control signs it off.
Price guide
| Work | Indicative London range |
|---|---|
| Single-storey rear extension | from £40,000 |
| Rear extension, per m² | £2,000 to £2,800 |
| Side return, per m² | £2,400 to £3,200 |
| Wraparound, per m² | £2,400 to £3,200 |
| Double storey, per m² | £1,800 to £2,600 |
| Basement extension | see the basement page |
Indicative London ranges (Architecture for London, Resi, MyBuilder and Checkatrade, 2026), not a quotation. Prices checked September 2026. Full breakdown: Extension Costs & Planning.
Quick estimate for Westminster
Westminster range: choose above
Rear and side-return extensions in Westminster
Westminster plots are tight and precious, so extensions here tend to reclaim every usable inch. On the Georgian and Victorian houses around Marylebone and Paddington, a rear closet wing and side return often become a single open kitchen-diner, carried on a steel beam and built off new foundations dug to suit the ground and any neighbouring structures. We survey drains, party walls and existing footings before a spade goes in.
Whether your project is permitted development or needs a full planning application depends entirely on the property. In a conservation area a single-storey rear extension can still be permitted development within the limits, unless an Article 4 direction removes it, but side, two-storey and roof extensions need an application to Westminster City Council. A listed building needs listed building consent for any extension, and flats have no permitted development at all. We advise honestly on which route applies, so you know the planning position before committing to a design.
Housing in Westminster
Roughly 132,230 dwellings sit on the valuation list for Westminster. 9% are houses and 91% flats, and by age 48% predate 1919, 10% are interwar and 40% post-war. Most homes here are flats, and a flat in Westminster has no householder permitted development rights, so extending one needs planning permission and the freeholder's consent.
Sizing a Westminster rear extension to what the guidance supports
Only 9% of Westminster homes are houses and 8% are terraced houses, so a house extension here is almost always an addition to a terrace. Westminster's conservation area guidance says full width rear extensions are generally not acceptable, except in certain circumstances at basement level, and that extensions must be architecturally subordinate to the main building. In a conservation area, then, the brief starts narrower than the plot: a part width addition that leaves the original rear wall readable, with the structural engineer we appoint planning the steel around the length of wall that stays.
Two further layers can apply. In Mayfair, Soho, Pimlico and Belgravia there are adopted neighbourhood plans, and applications in those areas are read against them as well as the City Plan. Read the one for your street before an architect draws anything. Westminster's householder pre-application service for an unlisted house or flat gives written advice only, on a 28 day timescale, which is quicker than learning the same lesson through a refusal. We price and programme from the design that comes out of that advice, not from the first sketch.
Planning and party walls in Westminster
Because so much of Westminster is protected, extension design is a conversation with the council as much as an engineering task. Your architect prepares a scheme that respects the stucco frontages, mansion-block symmetry and roofscapes the borough guards closely; we price and build it, and work with them on the planning application and, where relevant, listed building consent, alongside the building regulations submission for the structural work.
Party wall matters are almost unavoidable here. New foundations near a boundary and beams bearing onto shared walls trigger notices under the Party Wall Act, which you serve as building owner, and mansion-flat extensions in NW8 or W9 need the freeholder's licence to alter. We coordinate neighbours, freeholders and managing agents, then deliver the full build, from groundworks and steel through to roof, glazing and a complete interior fit-out.
Kitchen extensions in Westminster
91% of homes in Westminster are flats, and a flat has no householder permitted development, so a ground-floor flat's kitchen extension needs planning permission from Westminster City Council and the freeholder's consent. With so few houses here, the realistic kitchen gain in most Westminster homes is a better layout inside the flat rather than an extension. The fit-out is priced on its own page, kitchen renovation in Westminster; the build figures are in the table above. Double storey extensions belong to the few houses here.
- Single and double storey extensions
- Wrap-around extensions and infills
- Full fit-out, finishing and making good
VOA Council Tax stock of properties, March 2025, for all 24 statistical areas in Westminster. Counts rounded by the VOA.
Why Westminster Homeowners Choose GS Renovation
Homeowners across Westminster choose GS Renovation because an extension on a listed or conservation-area property demands more than a builder. We work with your architect on the planning application to the council, coordinate the engineer-designed steel and building control, then build the lot ourselves. Our founder's 30+ years on the tools means we know what this borough will and will not allow.
- Founder with 30+ years of hands-on experience
- Drainage diverted or built over with the right approval in place
- Insulation and thermal detailing to current regulations
Built a new extension at rear of property. Laid foundations, drainage, raised floor, new roof and completed all fixtures and interior decoration…
George always kept to commitments and arrived on time. He offered helpful suggestions during his initial visit and promptly provided a reasonably…
Saaid is the best craftsman I have worked with. He knows what he is doing from the very beginning. He has a great knowledge of construction…
FAQ
Frequently Asked Questions about House Extensions Westminster
Are there hidden costs with house extensions in Westminster?
Hidden costs on house extensions in Westminster come from what the W1 survey cannot see, not from the quotation, which sits within £2,000 to £2,800 per m² for a rear extension (indicative). Older property can uncover wiring, timber or drainage, and GS Renovation names what it suspects before starting. Approvals are never a surprise extra: permitted development or a planning application to Westminster City Council is priced from day one.
Will my Pimlico extension need planning permission or is it permitted development?
Many Pimlico houses sit within a conservation area or are listed. A conservation area takes away permitted development for side, two-storey and roof extensions, though a modest single-storey rear extension can still qualify within the limits, while a listed house needs listed building consent for any extension and usually a planning application to Westminster City Council. Elsewhere, a modest rear extension may fall under permitted development. We check your property's status first and tell you exactly which route your project needs.
Can a Westminster flat owner use permitted development for an extension?
No. Householder permitted development covers houses only, never flats or maisonettes, so enlarging a flat outward is a matter for a planning application to Westminster City Council. The freeholder's written consent under the lease is needed as well, and building regulations apply to the result. The VOA puts flats at 91% of the borough's dwellings.
What notices apply when digging foundations for a Westminster extension?
Terraced plots in NW8 or SW1 leave very little distance to the neighbouring footings, so the Party Wall Act excavation rule nearly always bites: any trench within 3 metres of their building that goes lower than its foundations needs a month's written warning. An objection hands the detail to surveyors, whose award sets the working conditions.
How will light reach the lower ground floor behind a new Westminster extension?
Mostly from above. Building out over a rear window leaves the original room deep inside the plan, so rooflights or a glazed lantern set tight against the old back wall do the work that window used to. Glass performs worse than an insulated roof, so the Part L insulation standard for the whole extension decides how much glazing is realistic, and that sum is done before the roof is drawn.
What does building control inspect on a Westminster house extension?
Every element that will later be buried. The inspector sees the foundation trenches open, the drains before backfill, the damp course, the steel bearings and the insulation before boards go over it. Where the drawings show the new walls crossing, or sitting close to, a public sewer, water company consent is secured before digging. The completion certificate follows the final visit.
When does a Westminster house extension become liable for the Community Infrastructure Levy?
Westminster charges CIL per square metre on new dwellings of any size and on any development where the internal area of a new building, extension or change of use exceeds 100 square metres. A modest rear addition is unlikely to reach that, but a scheme that adds several floors of space can. Relief exists for some householder extensions, but it has to be claimed before work starts. Add up the new internal area early so the levy is not a surprise.
Can a Westminster terrace have a full width rear extension?
Westminster's conservation area guidance says full width rear extensions are generally not acceptable, except in certain circumstances at basement level. With 56 conservation areas covering over 76% of the City, that guidance applies across a large part of Westminster. Plan for a part width addition from the start, and use the pre-application service if the site looks like one of the exceptions.
Does a Mayfair or Soho extension have to follow a neighbourhood plan?
Yes. Mayfair and Soho both have adopted neighbourhood plans, and applications in those areas must respond to their policies as well as the City Plan. The architect's design statement should show how the scheme meets both. We build to the approved drawings and conditions, so anything the neighbourhood plan adds, such as a materials requirement, is known before a price is fixed.
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