Extending a home in Mayfair is possible in theory and a long negotiation in practice. Listed buildings crowd every street. Conservation areas cover the ground between them. Most homes are flats with a freeholder above them. Anyone planning to add space here should budget for patience before bricks.
Historic England's register lists 2,989 buildings in and around Mayfair: 180 at Grade I, 327 at Grade II* and 2,482 at Grade II. Listing protects the whole building, inside and out, so changing a listed home needs listed building consent from Westminster City Council for works that would be routine elsewhere, from moving a partition wall to replacing a window. Grade II* and Grade I buildings draw closer scrutiny again, and a district with 180 of the top grade has more buildings of national importance than most cities hold.
The patch of central London around Mayfair contains 47 conservation areas, Mayfair's own among them. Conservation-area status gives the council extra control over demolition, alterations visible from the street and work to trees, so even an unlisted building is answering to the street it stands on. The streets were laid out by the Grosvenor, Berkeley, Burlington and Curzon estates in the first place, and the planners treat that layout as the finished article.
There are 3 scheduled monuments in the same wider area, the strictest tier of protection and relevant only to ground close to one.
People do alter homes here, constantly. Westminster logged 2,912 planning applications across the area between April 2024 and June 2026, 1,351 of them in the last twelve months. Each of those applications is somebody prepared to spend months on paperwork to change a building in Mayfair.
The ground adds its own condition. The British Geological Survey places the whole area in shrink-swell class 3, a published hazard class for clay that swells when wet and shrinks when dry. For an extension, and especially for a basement, that usually means deeper foundations and a structural engineer's report as part of the design. It is a building cost to plan for, and no reason to walk away.
Twenty listed buildings in the area are on Historic England's Heritage at Risk register. Those are buildings in poor condition whose repair may qualify for grant support and will certainly attract scrutiny, and they matter mainly to anyone buying one of them. For the owner of an ordinary flat nearby they are a sign of how closely the old fabric is watched.
Permitted development, the national allowance for small extensions without a planning application, does little here. About 90% of homes on the council-tax list are flats, which never had the householder allowances to begin with, and listed buildings and conservation areas narrow what remains for the few houses. Then there is the lease. A flat owner usually needs the freeholder's written consent for alterations as well as the council's, and on the old estates the freeholder has opinions of its own. Reading the lease's alterations clause is the practical check worth doing first, since the council is often the second body that has to say yes.
Freedom to extend is about as limited as it gets in Mayfair, and the space a home has on the day it is bought is usually the space it keeps.