Yes, for most houses, though a minority need permission from people with long memories. Dozens of buildings are listed. None of them is at the top grade. One conservation area falls within the area. The clay underneath has its own say as well. Most of Cookham can build out the back, and a few dozen owners have to ask history first.
Cookham has 64 listed buildings: none at Grade I, three at Grade II* and 61 at Grade II. Listing covers the whole building, inside and out. Almost any change that affects its character, from a new window to an internal wall, needs listed building consent on top of any planning permission, and doing the work without consent is a criminal offence, so it isn't just an administrative slip. For a village that had an abbey by the 8th century, Cookham has kept surprisingly little at the top grade.
One conservation area, Hedsor Road and Riversdale, falls inside the area's boundary box. Inside a conservation area, the council controls changes to the outside of a house more tightly than elsewhere. Some work that would be permitted development on an ordinary street needs an application here, and demolition needs consent. Find out whether a particular house is inside the boundary early on, because the council's map decides that, whatever the sales particulars say.
The planning records covering Cookham log 4,792 applications between April 2024 and June 2026, with 2,141 of them in the most recent twelve months. That is plenty of evidence that people here alter their homes, and alter them often. These figures count applications only. They don't show how many were approved. A parish that fought off attempts to enclose its common land in 1799, 1869 and 1928 is a parish that reads the planning list.
Every postcode in Cookham is in class 3 or above for shrink-swell clay on the British Geological Survey's scale. Clay like this expands when wet and shrinks when dry, and nearby trees make the effect stronger. That doesn't stop anyone extending. It means foundations may need to go deeper than the minimum, and a structural engineer should look at the ground before the architect draws anything ambitious. Ask the seller about any past subsidence claims or underpinning, because the answer affects both the extension and the insurance.
In practice, an ordinary house outside the conservation area and not listed has the normal permitted development rights, so a modest rear extension may need no application at all. Check the specifics with the council, since conditions on earlier consents can remove those rights. A listed house, or one inside Hedsor Road and Riversdale, needs consent for much more, and the process is slower. The same plan can be routine on one street and a long exchange of drawings on another.
Cookham sits in the middle for freedom to extend: most houses can be extended without fuss, while the listed minority and the clay underneath make sure nobody builds without thinking.