Yes, you can extend most houses in Denham, provided they aren't in the old village. There are 45 listed buildings, and almost all of them are grade II. The village has one conservation area, and it carries the village's own name. Applications arrive at a steady rate. The clay under the houses is the constraint nobody mentions in the sales pitch.
Historic England's list records 45 listed buildings in and around Denham. Two are grade I, one is grade II* and 42 are grade II. Owning any of them means getting listed building consent for most changes, inside and out, on top of any planning permission. Grade I and grade II* are the top tiers, which only a small minority of buildings reach. Grade II covers most of the list, and it still controls anything from the window frames to the chimney. Forty-five listings is a lot for a village, but most houses here aren't on the list. Owners of listed homes are generally aware of how much they have taken on.
The area has one conservation area, named Denham. Inside it, the council takes a closer interest in changes to the outside of a house, including what can be seen from the street, demolition, and some trees. Any extension inside it needs to look as though it belongs there.
People alter their homes here all the time. The council's planning register lists 503 applications covering Denham between April 2024 and June 2026, and 213 of them came in the most recent twelve months. That is a lot of paperwork for a village of about 2,000 people. It is good evidence that extending here is common, whatever the conservation officer thinks of the plans.
The British Geological Survey puts the ground under Denham at shrink-swell class 3, and all of the area reaches that class. Shrink-swell means clay that swells when wet and shrinks when dry, so a house built on it moves with the weather. For an extension, that means deeper foundations than a builder might quote at first, a structural engineer looking at the soil before the design is fixed, and care with large trees near the new walls. It adds to the cost and the planning, but it doesn't rule anything out.
What you can do depends mainly on where the house is. Permitted development rights let many single-storey rear extensions and loft conversions go ahead without a full application. Inside a conservation area those rights are narrower, and they never cover alterations to a listed building. A semi-detached house or a bungalow outside the conservation area has the most room to extend. A listed cottage in the old village has the least, and any plan there begins with the conservation officer. Check the council's conservation-area map and the national heritage list for the exact address. Most of the properties here are neither, and it is worth knowing for certain whether yours is one of them.
Freedom to extend in Denham is middling. Most homes can grow without much difficulty, the old village needs consent for nearly everything, and the clay matters for every build wherever the house is.