Yes, you can extend here, but expect to explain yourself. Most homes aren't listed. The old village core is a conservation area. Over half the area sits inside a protected landscape. The neighbours keep putting in applications, so it's clearly possible, though rarely quick.
Listing is the first thing to check. The Maresfield area has 77 listed buildings: two Grade I, five Grade II* and 70 Grade II. Listing covers the whole building, inside and out, so on a listed house even an internal wall or a replacement window can need listed building consent as well as planning permission. Grade II is the most common grade and the least strict, but that doesn't mean it's easy. The owner of a Grade II house who assumes that rules out any fuss usually learns otherwise at the first conversation with the conservation officer.
There's one conservation area, and it covers Maresfield itself. Inside it, the planning authority takes a closer interest in anything visible from the street, including roof changes, front extensions, walls and some trees. Changes that would count as permitted development elsewhere can need an application here. On top of that, about 56% of the area lies within the High Weald Area of Outstanding Natural Beauty, so even outside the conservation area the landscape affects what gets approved.
There are also three scheduled monuments nearby, the strictest tier of protection. They mostly matter if your land adjoins one. Anyone with a large garden near one of them should find out before sinking a foundation.
People still build here. Between April 2024 and June 2026 there were 1,193 planning applications in the area, 515 of them in the last twelve months. That's steady evidence that local homeowners keep extending and altering their houses and are willing to go through the process to do it. The volume doesn't tell you how many were approved. It tells you nobody treats the rules as a reason not to try.
In practice, check three things before you draw up plans: whether the house is listed, whether it sits inside the Maresfield conservation area, and whether it's in the protected landscape. If the answer to all three is no, ordinary permitted development may cover a modest rear extension. If any answer is yes, assume you'll need an application, a better-than-average architect and a few months of patience. Do those checks before you make an offer, not after the kitchen designer has already quoted.
Freedom to extend here is middling. A typical modern house outside the old core can be extended in the usual way. Older houses in the village centre come with a longer process and fewer guarantees.