Yes, you can extend a house in Deal, but plenty of people will want a say first. The town has hundreds of listed buildings. The neighbours apply for planning permission constantly. Under most of it lies the Kent coalfield. You're allowed to build here, as long as you've done the paperwork first.
Deal has 375 listed buildings. None is Grade I, 6 are Grade II* and 369 are Grade II. So none of them is officially a national treasure, but hundreds are protected as if they were. If your home is listed, almost any change needs listed building consent on top of planning permission: a new window, a knocked-through wall, sometimes the paint. The listing covers the inside as well as the front. No conservation area is mapped within the town's boundary, but that makes no difference to a listed house.
There are 4 scheduled monuments in the area, including the artillery castles at Deal and Walmer and the remains of Sandown Castle. That's the strictest protection there is. It mostly matters if your plot is next to one, and then even a garden shed gets looked at closely.
Plenty of people alter their homes anyway. Between April 2024 and June 2026 there were 1,153 planning applications across the area, 488 of them in the twelve months to June 2026. Every one of those was somebody wanting a bigger kitchen, a loft room or a garage turned into an office. They're only applications, and they say nothing about how many were approved. Deal is not a town that leaves its houses alone.
The mining past matters too. About 64% of postcodes lie within range of a recorded mine working, and 167 postcodes sit directly above one. The whole area is inside the coal authority's consultation zone, and Betteshanger gave its name to the largest colliery in the Kent coalfield. None of that makes a house unsafe. In practice it means your conveyancer orders a mining search, you may be asked for a specialist report, and lenders and insurers will want to see the result. Better to learn about the coalfield from the search than from your surveyor.
If a house isn't listed, modest extensions often fall under permitted development and don't need a full planning application, though the limits are tight and depend on the property. If it is listed, permitted development offers you almost nothing, and consent comes first. Check the listing status and the mining search before you make an offer, not after you've hired an architect.
Deal gives you less freedom to alter a home than most towns. You can do it, but with this many listed buildings and this much ground to check, expect to deal with consent forms and searches before you start.