Usually yes, as long as the house is one of the ordinary post-war homes and not one of the old ones. Heritage protection in and around Hazlemere is limited. A few dozen listed buildings and a handful of conservation areas cover a small part of a large suburb. Your neighbours are applying for planning permission at a steady rate. For most homes the main obstacles are the council and the ground, and history rarely comes into it.
There are 72 listed buildings in and around Hazlemere. One is Grade I, one is Grade II*, and the other 70 are Grade II. Listing protects the inside of a building as well as its front, so changing a window, a fireplace or the roof of a listed home needs listed building consent from the council as well as any planning permission. The 1960s and 1970s estates built over the Park and the old farms aren't in that group. Most of what was built in those decades is still free to change.
Four conservation areas fall in and around the area: Four Ashes, Penn and Tylers Green, Terriers and Totteridge. Inside them, the council looks more closely at changes that would be visible from the street, such as materials, a front extension or the loss of a tree, and some work that would normally go ahead without permission may need a proper application. Check Buckinghamshire Council's conservation area map before you buy, because you can't tell from the street which side of a boundary a house is on.
The neighbours are certainly building. Between April 2024 and June 2026, 2,352 planning applications were made in the area, 1,055 of them in the last twelve months. That's a lot of rear extensions, loft dormers and garage conversions, and a lot of people keeping an eye on the planning list to see what next door wants.
The ground needs a mention. The British Geological Survey's shrink-swell classification reaches class 3 at its worst here, the moderate band, and only a fraction of a percent of the area is at that level. Shrink-swell clay expands when wet and shrinks in dry summers, which can move shallow foundations. For most plots this makes little difference, but if your site is on one of those patches, the structural engineer may ask for deeper foundations than the builder's quote assumes.
For a typical house here, a modest rear extension or loft conversion can often go ahead under permitted development, the national rules that let homeowners do certain work without a full application. Those rights are narrower for listed buildings and can be tighter inside conservation areas. Building regulations still apply either way. Most people in Hazlemere don't need to fight a conservation officer, but they do need a good set of drawings.
Overall, your freedom to extend here is middling. Most of the post-war houses can be extended with ordinary care, while the older houses and the conservation areas come with more paperwork than a buyer might expect.