You can extend a house in Hammersmith, though there's a fair amount of heritage to get past first. More than 250 listed buildings stand in and around it. Six conservation areas cover the surrounding streets. Most homes are flats, and a flat comes with a freeholder before the council even gets involved. The neighbours file hundreds of planning applications a year anyway.
Historic England lists 254 buildings in and around Hammersmith: three at Grade I, 22 at Grade II* and 229 at Grade II. Listing covers the inside of a building as well as the outside, so knocking through a wall, replacing windows or changing a staircase in a listed house usually needs listed building consent as well as any planning permission. Doing the work without consent is a criminal offence, which tends to focus the mind before the builders turn up.
Six conservation areas overlap the area around Hammersmith. In a conservation area the council takes a closer interest in anything visible from the street, and some of the extensions that need no planning permission elsewhere need it here. It is the council's planning register, not the estate agent, that confirms whether a particular house falls inside one, so look it up on Hammersmith and Fulham's planning map while the purchase is still at an early stage.
People here extend anyway. Hammersmith and Fulham's planning records show 1,618 applications across Hammersmith between April 2024 and July 2026, 607 of them in the last twelve months. Those are applications, though, and an application tells you someone wants to build something, not that the council agreed. In a district with this much listed stock, a good share of them will be someone asking permission to change a window.
The ground is a separate issue. On the British Geological Survey's shrink-swell scale, every postcode in Hammersmith is class 3, meaning the clay underneath swells when wet and shrinks as it dries. That doesn't stop anyone extending. It does mean foundations usually go deeper than they would on firmer ground, and a structural engineer should see the plans before the building inspector does. Trees close to the house make the movement worse, so the engineer will want to know where they are as well.
Eight listed buildings in and around Hammersmith are on Historic England's Heritage at Risk register, which means they are in poor enough condition that repairs are both urgent and closely watched. Owners of those buildings may qualify for grant support and will certainly face scrutiny. For the buyer of an ordinary house nearby, they are a sign of how seriously the conservation officers take the district's older buildings, and nothing more.
In practice, the answer depends on what you buy. A freehold house outside a conservation area and off the listed register has the same permitted development rights as most houses in England, so a modest rear extension or loft conversion may need no planning application at all. Every one of those conditions narrows here. About three quarters of the housing is flats, and flats have no permitted development rights, so any extension needs planning permission and usually the freeholder's agreement too. The older the street and the closer it sits to the river, the more likely a conservation officer will want a say.
Your freedom to extend in Hammersmith is limited. The district's history is attached to a lot of its buildings, and anyone planning to change one should expect a long conversation with the council first.