Yes, you can extend a house in Chalfont St Giles, and on most streets it is an ordinary planning job, though in the old core it turns into a negotiation. Around fifty buildings in the village are listed. The historic centre is a conservation area. Neighbours file planning applications at a rate of hundreds a year. Underneath it all is ground that shrinks and swells with the weather, and that matters more to an extension than any of the paperwork.
Historic England lists 51 buildings in the area: two at grade I, two at grade II* and 47 at grade II. Almost any change to a listed building that affects its character needs listed building consent, and that covers the inside as well as the outside, so replacing a staircase can need permission just as an extension would. Most of the village's homes aren't on that list. For the 51 that are, the house belongs to you and its character is protected by law.
The area has one conservation area, named after the village itself. Inside it, Buckinghamshire Council looks harder at anything that changes how a street looks, and works that would be routine elsewhere can need an application. Check Buckinghamshire Council's conservation area map to see whether the house you are looking at is inside the boundary, because neighbouring houses on the same road can fall on different sides of it.
Residents have certainly not given up on building. Buckinghamshire Council's records show 1,733 planning applications in the area between April 2024 and June 2026, 730 of them in the twelve months to June 2026. Those are applications made, not permissions granted. Some of these will be for trees and minor works, though, and the record shows that altering homes here is a normal, busy business. In Chalfont St Giles, many of your neighbours will have their own planning history, which is useful when you are about to start yours.
The British Geological Survey puts the whole area in class 3 for shrink-swell ground, which means clay that expands when wet and shrinks when dry. That is a published geological hazard class, and it is a practical point rather than a reason to panic. Extensions on this kind of ground usually need deeper foundations, and nearby trees can affect how the soil behaves, so a structural engineer's view on the foundation design will save money that a cheap quote would lose. Every house in the village sits on that class of ground, inside the conservation area or not.
In practice, a house outside the conservation area and not listed can often use permitted development for modest rear extensions, subject to the national limits and the council's view where prior approval applies. Inside the conservation area, some of those rights are narrower, especially for anything visible from the road. A listed house needs consent for nearly everything, and a builder who says otherwise is not the person whose signature counts. The same project can be routine on one road and need a full application on the next, so find out which kind of house you are buying first.
How freely you can extend in Chalfont St Giles is middling: straightforward across most of the newer housing, tightly controlled in the listed core and the conservation area, and on clay everywhere that needs proper foundations.