Yes, you can extend in Dorking, provided you don't mind paperwork. Well over a hundred buildings are listed. Three conservation areas cover the old centre and its edges. Green Belt and protected hills wrap round the town. And the neighbours still send in hundreds of planning applications a year. Planning here is a queue, not a wall.
Dorking has 157 listed buildings. None is Grade I, 5 are Grade II* and 152 are Grade II. Grade II is the most common tier nationally and the least restrictive, but it still means listed building consent for almost any change that affects the building's character, inside or out, and that includes the windows, the staircase and the fireplace you were planning to open up. If the house you're viewing is listed, the extension conversation starts with what the council will let you keep, not what you want to add. Unauthorised works to a listed building are a criminal matter, not a planning dispute.
There are three conservation areas: Dorking itself, Dorking Pixham Lane and Milton Street. Being inside one doesn't stop you building, but it tightens the rules on demolition, on trees and on anything the street can see, and the council will judge a side extension or a new dormer by whether it fits the area, not just whether it fits the plot. Owners in the town centre learn the phrase "in keeping" fairly early.
There are also three scheduled monuments, the strictest tier of protection, including a Bronze Age bowl barrow in The Glory Wood. Unless your garden backs onto one, they won't touch your plans.
Nobody is being put off. Mole Valley's planning records hold 2,041 applications from April 2024 to June 2026, with 791 in the last twelve months alone. That's a town that is extending, converting and replastering steadily, conservation areas or not. The volume says people apply. It doesn't say how many succeed, and nothing here should be read as an approval rate.
The ground needs a look too. The British Geological Survey puts almost all of the area, about 99%, in shrink-swell class 3, its moderate hazard class for clay soils that expand when wet and shrink when dry. For an extension that means deeper foundations than you might expect, and trees near the footprint need thinking about. It's a reason to get a structural engineer's view before you sign off the design, not a reason to abandon the project.
In practice, a rear extension on an unlisted post-war semi outside the conservation areas may fall within permitted development, which lets certain works go ahead without a full application. A listed house, anything in a conservation area and anything visible from the street should be assumed to need consent until the council says otherwise. The town is also surrounded by Metropolitan Green Belt and bordered on three sides by the Surrey Hills National Landscape, and homes on the edges face tighter rules on anything that makes a building larger. Check the property's status with Mole Valley before you budget for the kitchen, not after the architect has drawn it.
Freedom to extend in Dorking is on the restricted side: very possible on the ordinary post-war streets, slow and conditional anywhere the town's history or its hills are involved, which is most of the places buyers want.