You can, but you'll be asking permission more often than in most towns. Nearly three hundred buildings are listed. Three conservation areas cover the old stone streets. Even so, the neighbours send the council hundreds of applications. In Corsham, an extension is a correspondence with the council before it's a building project.
Corsham has 287 listed buildings: 4 at Grade I, 14 at Grade II* and 269 at Grade II. Grade I and II* are the top tiers, for buildings of exceptional or more-than-special interest, and changes to them get the closest scrutiny. Grade II covers the rest, and that's still a legal protection, not a compliment. If your house is listed, work that affects its character generally needs listed building consent on top of any planning permission, and that can mean the inside as well as the outside. Plenty of buyers see the listing as part of the charm. It stays charming right up until the day they want the wall down.
Three conservation areas cover Corsham, Neston and Pickwick. Inside them, the council cares about how the street looks as a whole, so changes that would pass without comment elsewhere, such as work to the front of a house, demolition, or a larger extension, are more likely to need a formal application. The rules follow the address, and buyers who assume they only follow listed buildings tend to learn this from a letter.
There are 8 scheduled monuments in and around the town, the strictest tier of protection, which mainly matters if your land sits next to one. Corsham's list is stranger than most. It includes rooms inside the Ministry of Defence's underground quarry complex, so the neighbours you need to worry about aren't always on the surface.
Plenty of people make changes anyway. The council received 461 planning applications in the area between April 2024 and June 2026, and 209 of them came in the last twelve months. That's hundreds of households choosing paperwork over living with the kitchen they bought. An old town with a lot of protection doesn't mean nobody builds. It means everybody applies.
The ground needs its own survey. The British Geological Survey's highest shrink-swell class across Corsham is class 3, and about 49% of the area's postcodes reach it. Shrink-swell clay expands when it's wet and shrinks when it's dry, and foundations laid in it need to allow for that movement. For an extension, that means a structural engineer's view on foundation depth before the drawings are final, especially near trees. It's a routine design point, not a reason to walk away, but it is a reason to pay for proper advice.
In practice, a post-war house outside the conservation areas and off the listed register may be able to use permitted development rights for modest work, the same as most of England, and the planning portal is the place to confirm that before a builder quotes. A listed house, or one inside Corsham, Neston or Pickwick's conservation areas, should be treated as needing consent until the council says otherwise. Checking which kind of house you're buying costs nothing before you exchange contracts. Finding out afterwards can cost the extension.
Freedom to extend or alter is one of Corsham's weaker points. Newer houses on the edges have room to grow, but across the old town the protections outweigh the owner's plans, and most ambitions for the house will be settled at the council.