Yes, for most houses, and with a lot more paperwork for the old ones. Marlow has more than 170 listed buildings. The old town is a conservation area. The neighbours lodge hundreds of planning applications a year. Extending here is normal; extending without asking is the part that goes wrong.
Historic England lists 174 buildings in Marlow: three at Grade I, 19 at Grade II* and 152 at Grade II. Listing covers the whole building, inside and out, so on a listed home almost any change that affects its character, from windows to internal walls, needs listed building consent from the council on top of any planning permission. Grade II* and Grade I homes face closer scrutiny again. If the house you want is on that list, you are buying it with a permanent co-owner who has no money in it and a great many views.
The area has one conservation area, covering the old town of Marlow. Inside it, demolition, some extensions and changes to the front of a house can need consent that an identical house outside the boundary would not. The town's oldest streets are inside it, and so is most of what people picture when they say they want to live in Marlow.
Buckinghamshire Council received 1,185 planning applications in the area between April 2024 and June 2026, 503 of them in the most recent twelve months. That is plenty of residents wanting a bigger kitchen or a loft room, and every one of them filled in the same form. The volume says nothing about how many succeeded, but it does say that altering a house here is a well-trodden process, and that the neighbours will see your application on the same list theirs went on.
The British Geological Survey puts the whole area in class 3 for shrink-swell ground, the clay that swells when wet and shrinks when dry, and the same class for soluble rock that can dissolve and leave voids. Neither is a reason to give up on an extension, but both mean foundations may need to go deeper than a builder's standard quote assumes, and a structural engineer's view belongs at the start of the project. Ground conditions are the item most likely to change the budget after the drawings are already done.
For an ordinary unlisted house outside the conservation area, many rear extensions and loft conversions can go ahead under permitted development, the national rules that allow some work without a full application, though conditions and size limits apply. Listed homes have no such route, and homes inside the conservation area have a narrower one. The one check worth making before committing to a house you plan to extend is to ask the council whether it is listed or inside the conservation area, and to get the answer in writing. A builder saying "that'll be fine" carries no legal weight at all.
Marlow gives a middling amount of freedom. Most of the post-war town can be extended with the usual permissions, but the old centre and its listed buildings come with rules that turn a simple project into a negotiation.