House Extensions Mayfair
Single storey rear and double storey extensions in Mayfair, 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 builds house extensions across Mayfair and the wider W1 district, adding space to Georgian and Regency townhouses close to Hyde Park and the West End. We handle groundworks, steel, roofing and fit-out, and guide you through Westminster City Council's planning and conservation requirements.
In Mayfair, 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 Mayfair: 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 Mayfair
Mayfair range: choose above
Rear and side-return extensions in Mayfair
Plots in Mayfair are generally tight and hemmed in by neighbouring townhouses, so extensions here tend to be modest rear or side-return additions, lower-ground enlargements or roof-level works rather than sprawling ground-floor builds. We assess the foundations, dig and underpin where the party wall and existing structure demand it, and tie new steel into the original brickwork so the extension carries its loads correctly and safely.
Most Mayfair homes are flats, which have no householder permitted development, and many of the houses are listed, which brings listed building consent into any extension. A whole, unlisted house in a conservation area keeps limited rights for a single-storey rear extension unless an Article 4 direction removes them, but side, two-storey and roof extensions need planning permission. That means most extensions here go through a full planning application to the council rather than permitted development. We advise honestly on which route your property qualifies for before any drawings are commissioned, so there are no surprises later in the process.
Housing in Mayfair
The Strand, St James & Mayfair statistical area counts roughly 7,560 homes for Council Tax; 52% are older than 1919, 5% interwar and 41% post-war, with 96% flats and 4% houses. A flat in Mayfair has no householder permitted development rights, so extending one needs planning permission and the freeholder's consent, which matters here because most homes are flats.
What Westminster's design guidance allows behind a Mayfair house
With 4% of Mayfair homes being houses, an extension here is an uncommon project, and it is judged against guidance written for historic streets. Westminster's Development and Demolition in Conservation Areas guidance says extensions should respect the scale and proportions of the existing building and must be architecturally subordinate to the main building. It also says full width rear extensions are generally not acceptable, except in certain circumstances at basement level. For a Mayfair house that steers the design towards a narrower addition that leaves part of the original rear wall readable, or towards space below ground.
Two documents shape the application beyond that guidance. The Mayfair Neighbourhood Plan was adopted on 24 December 2019, and applications in the area must respond to its policies as well as to the City Plan, so the design statement should address both by name. Westminster's householder pre-application service gives written advice only, on a 28 day timescale, for alterations and extensions to an unlisted house. Booking it before the architect's scheme is fixed means the officer's view shapes the drawings rather than a refusal reshaping them, and the build programme can start from a design with a realistic chance of approval.
Extending period townhouses across Mayfair
A typical Mayfair extension project runs from foundations and drainage through steel beams, roofing and a complete internal fit-out, all managed by one team. We match new brick and stone to the period elevation, replicate cornices and joinery inside, and detail the roof to suit the building's proportions. Where the extension builds on or against a party wall, or its foundations are dug close to next door, the Party Wall Act applies: you serve the notices as building owner (or your surveyor does) before any excavation begins, and we build to any award.
Working in W1 means coordinating deliveries and skips on busy Westminster streets near Oxford Street and the West End, so we plan logistics tightly to keep disruption to your neighbours low. From the first structural survey to the final coat of paint, we sequence the trades so your extended kitchen-diner or additional bedroom is delivered as a single, accountable contract rather than a string of separate jobs.
Kitchen extensions in Mayfair
96% of homes in Mayfair 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 Mayfair homes is a better layout inside the flat rather than an extension. The fit-out is priced on its own page, kitchen renovation in Mayfair; the build figures are in the table above. Double storey extensions belong to the few houses here.
- Steel goalpost frames where the rear wall opens up
- Party wall process explained, and the extension built to the award
- Building control inspections at foundation, damp course and steel stages
VOA Council Tax stock of properties, March 2025, for the Strand, St James & Mayfair statistical area, which also covers St James's and the Strand. Counts rounded by the VOA.
Why Mayfair Homeowners Choose GS Renovation
Mayfair extensions are constrained by tight plots, listed status and strict Westminster planning, so experience matters. GS Renovation manages the planning route, structural steel and full fit-out under one roof, matching new work to the period fabric. With our founder's 30+ years on the tools, we deliver additional space that respects the character of your home and the conservation area around it.
- Founder with 30+ years of hands-on experience
- Foundation depth set after trial holes, drains and trees are checked
- Programme planned around party wall notice periods before excavation near a neighbour
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 Mayfair
How do you price house extensions in Mayfair?
The method for pricing house extensions in Mayfair is deliberately dull: measure, scope, itemise, then compare with £2,000 to £2,800 per m² for a rear extension (indicative). Nothing is quoted over the phone, because a blind figure changes later. GS Renovation allows for the period construction at W1 and prices permitted development or a planning application to Westminster City Council as its own line.
Can I extend my Mayfair home under permitted development?
Usually not. Flats have no householder permitted development, and most Mayfair homes are flats. For a whole house in a Westminster 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 planning permission, and a listed house needs listed building consent for any extension. We assess your specific address first and advise on the correct route before any design work starts.
Must neighbours be told before foundations are dug for a Mayfair extension?
Almost always. A footing trench that runs closer than 3 metres to next door and bottoms out below their foundations falls under the 1996 Act's excavation rules, which ask for a month's written warning. Plots here are so hemmed in that few trenches escape it. If the owner objects, surveyors take over and set the method, the timing and any monitoring of the adjoining structure.
What if a sewer runs under the ground where my Mayfair extension would go?
Then the water company has a say before anything is built. Building over or close to a public sewer needs their consent, and the design may have to bridge the pipe or leave access to it. A private drain can usually be diverted, laid to proper falls under Part H. Surveying the drains by camera before the footprint is fixed avoids redrawing the scheme later.
What insulation standard does a Mayfair extension have to reach?
The new work is judged against today's Part L, however old the house it adjoins. Those targets fix how thick walls, roof and slab become, and they cap the glass area before the heat loss sums stop adding up. For a lower-ground extension, the insulation and damp-proofing below ground are detailed together, and building control inspects both before they are hidden.
How is the solid rear wall of a period Mayfair building opened into an extension?
With an engineer's steel and careful temporary works. Walls in the pre-1919 buildings that make up 52% of Mayfair's stock are solid brick, so needles are passed through above the opening on props while the beam goes in on padstones. The opening is formed only after the extension shell is weathertight, and building control sees the bearings before they are boxed in.
Will a Mayfair house extension be liable for the Community Infrastructure Levy?
Only above a size threshold, unless it creates a new home. Westminster's charging schedule first came into effect on 1 May 2016, and it charges per square metre on new dwellings of any size and on any development where the internal area of an extension exceeds 100 square metres. A Mayfair scheme combining a rear addition with a storey dug below ground can pass that line. National rules let a homeowner claim relief for their own extension, but only if the claim is made before work starts, so settle it with the planning application.
Does Westminster's Code of Construction Practice apply to a Mayfair extension?
It applies if the extension includes basement development or counts as a major development site. The code covers those schemes where consented from September 2016 onwards, imposed through a pre-commencement planning condition, so logistics and neighbour arrangements are approved before work starts. An above-ground extension that is neither falls outside that code, though its planning consent may carry its own conditions on hours, deliveries or construction management.
Why might a full width addition behind a Mayfair house work below ground but not above?
Because Westminster's conservation area guidance treats the two differently. It says full width rear extensions are generally not acceptable, except in certain circumstances at basement level. Going down, though, brings the city-wide Basement Development Article 4 Direction into play, in force since 31 July 2016, so the basement part always needs planning permission. The trade is a less visible extension in exchange for a heavier structural and planning process.
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