You can, but in parts of Ewell you will need permission and patience. Hundreds of listed buildings sit in and around the town. Conservation areas cover the old village and a good deal beyond it. The clay underneath has its own opinion about foundations. In the right street the extension gets built, and in the wrong one it becomes a long correspondence with the council.
There are 253 listed buildings in and around Ewell. None is Grade I, 12 are Grade II* and 241 are Grade II. Listing protects the whole building, inside and out, so even modest changes to a listed house need listed building consent from Epsom and Ewell Borough Council on top of any planning permission. Ewell's heritage has no single famous landmark, just a large number of ordinary-looking protected houses.
The same area holds 19 conservation areas, among them Ewell Village, Church Street and Ewell Downs Road. In a conservation area the council takes a closer interest in anything visible from the street: extensions, demolition, cladding and sometimes the trees in your own garden. Whether any extra local restrictions apply varies from one area to the next, so check the council's conservation-area map for the exact house early on, because the boundary can run between neighbouring roads.
Three scheduled monuments sit in and around the town, including the tower of Ewell's old church, which stands on its own in parkland. Scheduling is the strictest form of protection, and it mainly matters if your plans involve digging on land next to one.
None of this has stopped people trying. Councils covering Ewell logged 3,313 planning applications between April 2024 and June 2026, 1,418 of them in the latest twelve months. Interwar semis are the most common house here, and the owners clearly do not intend to keep them at their 1930s size.
The British Geological Survey puts shrink-swell risk across about 100% of the area at class 3, which means clay ground that swells when wet and shrinks when dry. Ewell grew up where the chalk meets the London clay, and an extension built on that clay needs foundations dug to suit it. A structural engineer will usually want to look at the ground and any nearby trees, and the foundations may have to go deeper than a builder's first quote allows for.
Outside the conservation areas, an unlisted interwar house can often take a modest rear extension or a loft conversion under permitted development rights, without a full planning application. Inside a conservation area those rights are narrower, and on a listed house almost every change needs consent. That leaves an easy half of Ewell and a difficult half, and the dividing line is a council map, not anything you can see from the street.
Freedom to extend or alter a home is one of Ewell's weakest points. The protected buildings and areas are everywhere, the clay adds cost underground, and only the unlisted streets outside the conservation areas are straightforward.