You can extend a house in Balham, though across much of it you will be asking permission first. Conservation areas cover large parts of the streets in and around it. Listed buildings are scattered through them. The clay underneath moves with the weather. The neighbours apply to build things all the time, so it can be done, with paperwork.
Historic England lists 52 buildings in the area. None is grade I, one is grade II*, and the other 51 are grade II. Any change to a listed building that affects its character, inside or out, needs listed building consent on top of any planning permission, and that covers the dull parts as well as the grand ones: windows, internal walls, a staircase. Owning one of the 51 means owning a house with a second, unelected occupant who has views on the joinery.
There are 14 conservation areas in and around the district, among them Abbeville Road, Hyde Farm, Culverden Road, Dinsmore Road, Clapham Park and Northbourne Road, and the Heaver Estate, the substantial houses the Victorian builder Alfred Heaver put up on the grounds of Bedford Hill House. Inside a conservation area, the council expects changes to respect the street's existing character, and works that would be routine elsewhere, such as altering the roof, cladding the front or taking down a tree, can need consent or notice first. On the Heaver Estate, a Victorian builder's taste in front elevations still outranks yours. Wandsworth's conservation-area maps show exactly which side of each boundary a property falls, and it is the first thing to look up on any house here, because neighbouring streets can sit under different rules.
The neighbours are not deterred. The council's planning register holds 1,070 applications in the area between April 2024 and June 2026, and 481 in the last twelve months alone. That is a street full of people who have been through the process, and every one of them will read your application with the close attention of someone who had to wait for theirs.
The ground needs respect too. The British Geological Survey puts Balham in class 3 for shrink-swell clay, and the whole area reaches that class. Clay at that level swells when it is wet and shrinks when it dries, so an extension here needs foundations deep enough to sit below the movement, and trees nearby make the calculation harder. A structural engineer's view on foundation depth belongs in the budget from the start, and the building inspector will ask for it anyway.
In practice, a house outside a conservation area and not listed keeps the usual householder allowances for modest rear extensions and loft work, subject to the national limits. Inside a conservation area, those allowances shrink and more work needs a full application. A listed building needs consent for almost anything. And since most homes in Balham are flats, most residents start without the householder allowances at all, so for them nearly any change to the outside goes through the council.
Freedom to extend in Balham is limited. The work happens, as the neighbours prove, but it happens with consent, with an engineer, and with time built in.