Yes, in most of Leatherhead, though the old centre and the clay underneath both have something to say about it. The town has a long list of listed buildings, though none at the top grade. One conservation area covers the historic core. Every postcode sits on clay that moves. Outside the High Street, your neighbours are extending all the time.
Historic England lists 99 buildings in and around Leatherhead. None are Grade I, five are Grade II* and 94 are Grade II. Owning one means listed building consent for almost any change to the fabric, inside as well as out, and that covers windows, walls and the staircase. Doing the work without consent is a criminal offence, and the council can make the owner put it back.
The town has one conservation area, covering Leatherhead's historic centre. Inside it, demolition, some changes to front elevations and work to trees need extra permission, and the council takes a closer interest in materials and appearance. Outside it, the rules are those that apply to most English suburbs.
There are five scheduled monuments in the surrounding area, the strictest protection there is. One is a group of bowl barrows on Leatherhead Downs, part of a round barrow cemetery, which mainly matters to anyone whose land backs onto one.
Mole Valley's planning register holds 1,640 applications for the Leatherhead area between April 2024 and July 2026, 607 of them in the last twelve months. Those are householders, landlords and businesses asking to change things, and they make it clear that alterations here are routine. Thorncroft Manor's records survive almost complete from 1278, so the town has been keeping a careful note of what people do to their property for a long time. Checking any address's planning history on the council's register, and whether it falls inside the conservation area, takes ten minutes and shows what the previous owners tried.
The British Geological Survey gives Leatherhead's ground a shrink-swell class of 3, and every postcode in the area reaches that class. Clay at that level swells when wet and shrinks when dry, so an extension needs foundations deep enough to sit below the movement, and building near large trees needs extra care. About 12% of postcodes also reach class 3 for soluble rocks, the chalk side of the town. A structural engineer's view and a building-control conversation about foundation depth will be part of any extension budget. Clay does not stop people building here, but it decides how deep the foundations go.
For a house outside the conservation area and not listed, many single-storey rear extensions, loft conversions and outbuildings can go ahead under permitted development, within national size limits, without a full planning application. Flats have no householder permitted development rights at all, which matters in a town where so many homes are flats. Anything bigger, anything on a listed building and much of what changes the front of a house in the conservation area will need consent from Mole Valley District Council.
Your freedom to extend in Leatherhead is middling. The listed buildings and the conservation area cover a small, old part of the town, and the rest is ordinary suburban planning on ground that needs proper foundations.