You can, but the building is only one of the parties involved. The area holds well over a thousand listed buildings. Twenty-one conservation areas sit inside it. So do more than a hundred scheduled monuments. Extending here is less a construction project than a negotiation with several institutions that got here first and have no intention of leaving.
Start with the listings, because they are the constraint most likely to touch an ordinary house. Historic England's register records 1,112 listed buildings across the area, of which 19 are grade one, 42 are grade two star and 1,051 are grade two. That last number is the one that matters to buyers. Grade two is the ordinary tier, and ordinary here means over a thousand homes where changing a window is a consent application rather than a Saturday. Listing covers the whole building, inside as well as out, including later additions, and listed building consent sits alongside planning permission rather than replacing it. Carrying out work without it is a criminal offence, not a fine to be shrugged at, which is why the sensible order of operations is architect, then conservation officer, then builder.
Conservation areas add a second layer over whole streets rather than individual buildings. There are 21 within the area, which brings controls over demolition, work to trees and the general appearance of the street scene, and means the materials you use are a matter of public interest rather than personal taste. A house does not need to be listed for its front elevation to be somebody else's business.
Then there are 144 scheduled monuments, the strictest protection in the system. Most of them are the barrows, earthworks and field systems scattered across the heath and downland rather than anything in a garden, but if a monument sits on or near your land, consent runs through a national process and the question stops being about design. It becomes about whether the work happens at all.
None of which means nobody builds anything. Between April 2024 and July 2026 the local planning register recorded 3,305 applications across the area, with 1,367 of those in the most recent twelve months. People here alter, extend and convert constantly. They simply do it through the front door of the planning system rather than round the back.
Two ground issues are worth knowing before an architect draws anything. On mining legacy, about 4% of postcodes in the area are affected by a recorded working, and one postcode sits directly above one. That does not mean a property is unsafe or that anything is going to move. It means a mining search is worth doing, a specialist report may be asked for, and a lender or insurer may want to see both before they commit.
The more widespread issue is shrink-swell ground. The British Geological Survey puts the area at class three at its highest, and about 84% of it reaches class three or above, meaning the clay content is enough to move with moisture. For an extension this shows up as foundation depth, drainage design and a structural engineer's opinion on nearby trees. It is a design input and a cost, not a reason to walk away, and it is far cheaper to find out at drawing stage than at building control stage.
Practically, assume less permitted development freedom than you would get elsewhere and check the specific property's status before making an offer conditional on plans. A pre-application conversation with the conservation officer costs a fraction of a redesign. Budget for measured drawings, a heritage statement where the building warrants one, and a longer timeline than a builder will quote you.
Freedom to alter a home is the weakest thing about this area by a distance. The heritage density that makes the place worth photographing is the same density that turns a rear extension into a project with a case officer, and buyers who want to reshape a house should be honest with themselves about which of those two they are actually buying.