Yes, you can extend a house in Mortlake, but expect it to take longer and need more paperwork than you might hope. The district has dozens of listed buildings. Fourteen conservation areas cover parts of it. The clay underneath shrinks and swells across the whole area. Hundreds of neighbours apply for planning permission every year anyway, so it can be done, though rarely quickly.
There are 49 listed buildings in the area: no Grade I, three Grade II* and 46 Grade II. Listing protects the whole building, inside and out, so almost any change that affects its character needs listed building consent on top of planning permission, even work that would otherwise need no permission at all. Doing the work without consent is a criminal offence, not just a planning problem. If the house you are buying is one of the 49, every alteration starts with that consent.
Fourteen conservation areas cover parts of Mortlake and the streets around it. They include Mortlake, Mortlake Green, Model Cottages, Cowley Road, Christ Church Road and East Sheen Avenue. In a conservation area the council controls more of what happens to the outside of a house: changes to the front, demolition, roof alterations and some side extensions all get extra scrutiny, and work that is automatic elsewhere may need an application here. Mortlake was Britain's leading maker of tapestry for most of the seventeenth century, and the council still checks every change to the street scene with the same care.
Plenty of people apply anyway. The council received 765 planning applications in the area between April 2024 and June 2026, 331 of them in the last twelve months. That is a lot of rear extensions, loft conversions and new windows going through the process, and it shows that homes here do get altered. It is also worth looking at the applications already made on your street, because they show which kinds of extension your neighbours have already put to the council.
The ground needs attention. The British Geological Survey puts Mortlake's shrink-swell hazard at class 3, and the whole area reaches that level. In practice this means clay soil that shrinks in dry summers and swells when it is wet, which can move shallow foundations. Any extension here will need foundations designed for clay, often deeper than standard, and a structural engineer should be involved from the start, not called in once the walls are cracking. The clay will not stop an extension, but it does make one more expensive.
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 grants and will be watched closely. It matters only to whoever owns that building, not to the buyer of an ordinary house down the road.
For a house outside the conservation areas and not listed, the national permitted development rules may cover a modest rear extension or loft conversion without a full planning application. Mortlake has so many conservation areas, though, that it is unwise to assume your house is outside one. The council's planning department can confirm which rules apply to a specific address, and a lawful development certificate turns that confirmation into a document you can rely on.
Freedom to extend is limited in Mortlake: the planning controls and the clay both make any project slower and more expensive than the same work in a less protected suburb. It can be done, but for anyone wanting a quick, simple extension, the easiest house here is one that already has it.