Mostly yes, unless the house is one of the old ones near the ponds. More than a hundred buildings are listed. A conservation area covers the old village. The neighbours put in hundreds of planning applications a year. Almost every house is built on clay. For most extensions, the ground under the house is a bigger obstacle than the council.
Historic England lists 135 buildings in and around Carshalton: none at Grade I, 6 at Grade II* and 129 at Grade II. For a listed building, almost any change to its character needs listed building consent on top of any planning permission, and that covers the inside as well as the outside. Grade II* means the building is of more than special interest. Most of the 135 are Grade II, but even at Grade II, swapping a sash window can mean filling in an application form.
Historic England's records count one conservation area within the area. Locally, that is the Carshalton Village Conservation Area around the ponds and the High Street, which was extended in 1993 to take in parts of Mill Lane, The Square and Talbot Road. The council also designates conservation areas at Wrythe Green and Park Hill. Inside a conservation area, demolition, some extensions and changes to the front of a house get more scrutiny than they would on an ordinary street.
There are also 2 scheduled monuments in the area, which have the strictest protection of all. They mainly matter to anyone with land near one.
People do extend here. The council received 746 planning applications from April 2024 to June 2026, and 324 of them came in the last twelve months. Much of that is the interwar semis and terraces adding lofts and rear extensions. To check the house you are looking at, search its planning history on the council's website and ask the council whether it is listed, locally listed or inside a conservation area. Each of those changes what you can do.
The ground needs more care than the planning rules. The British Geological Survey rates shrink-swell clay in Carshalton at class 3, and about 99% of the area falls in that class or above. In practice, an extension will probably need deeper foundations than on firmer ground, and a structural engineer should look at it before the builders arrive. Large trees near the footings make the clay worse. Even a straightforward extension here costs more below ground than above it.
One listed building in the area is on Historic England's Heritage at Risk register. That means a building in poor condition, whose repair may qualify for grant funding and will be watched closely. It affects whoever owns that building, not the semi down the road.
For an ordinary unlisted house outside a conservation area, many rear extensions and loft conversions can go ahead under permitted development rights, without a full planning application. A listed building, or a house inside the village conservation area, usually needs consent for a good deal of what would be allowed automatically elsewhere. Those are the houses people move to Carshalton for, so most buyers want them and many will find the rules strict.
Freedom to extend is middling here. Outside the old village it is fairly open, inside it the rules are close, and the clay pushes up the cost of building anywhere in town.