You can extend a house in St John's Wood, in the sense that you can apply. Hundreds of buildings are listed. Seventeen conservation areas cover the district and its edges. The ground underneath is clay that moves. An extension here gets designed twice, once by you and once by the council.
The listing count is the first thing to know. There are 655 listed buildings in and around St John's Wood: 11 at Grade I, 21 at Grade II* and 623 at Grade II. Listing protects the whole building, inside and out, so changes that would be routine elsewhere, a new window, an internal wall, a rear addition, need listed building consent on top of any planning permission. The Grade II houses carry most of that load, and the council treats them as protected buildings first and family homes second.
Conservation areas widen the net. The district's map takes in 17 of them, among them St John's Wood itself, Maida Vale, Elsworthy, Eton, Alexandra Road and Lisson Grove. Inside a conservation area the council takes a closer interest in anything that changes how a street looks, from roof extensions to front boundaries, and in practice that means more applications and more scrutiny. With seventeen of them in one district, a buyer is rarely more than a street away from the next set of rules.
The neighbours are not deterred. There were 1,811 planning applications across St John's Wood between April 2024 and June 2026, and 795 of them in the last twelve months of that window. Residents here clearly do alter their homes. They just do it with drawings, consultants and a lot of patience, and the planning list is probably the most closely read document in the district.
The ground adds its own condition. The British Geological Survey puts St John's Wood at class 3 for shrink-swell, and 100% of the area reaches that class. Clay of this kind swells when wet and shrinks when dry, which matters most for new foundations and for anything built near trees. An extension here usually means deeper foundations and a structural engineer's view at the design stage, and that line belongs in the budget from the start.
Eight listed buildings in the area are on Historic England's Heritage at Risk register. Those are buildings in poor condition, where repairs may attract grant support and will certainly attract scrutiny. For the owner of an ordinary flat or house nearby they are not a hazard, though in a district this expensive, getting a building onto a neglect register takes some doing.
The practical position is that many of the small changes allowed without an application elsewhere need permission here, because listing and conservation-area status take much of that freedom away. Look up the property on Westminster's planning register, check whether it is listed, which conservation area it falls in, and what earlier owners applied for and were told. A refused application from five years ago is the most useful document a buyer can find.
Your freedom to extend or alter a home in St John's Wood is poor. Buy the space you need on the day you buy, because adding it later means the listing rules, the conservation officer and the clay all getting a say.