Yes, and plenty of the neighbours are already doing it. The district has very few listed buildings. A handful of conservation areas sit in and around it. Planning applications go in by the hundred every year. The council is the easy part. The clay under the house deserves more attention.
Historic England lists 21 buildings in Thornton Heath: one at Grade I, one at Grade II* and 19 at Grade II. Listing means listed building consent for almost any change inside or out, and the Grade I and II* buildings are guarded hardest of all. Across a district of about 24,000 homes on the council-tax list, very few people will ever own one. For almost every buyer, listing is something that applies to other people's buildings.
Six conservation areas fall in and around the district: Beulah Hill, Norbury Estate, Harold Road, Church Street, Central Croydon and Wellesley Road North. Inside one, changes to the front of a house, its windows, its roofline and anything visible from the street get closer scrutiny, and some permitted development rights can be taken away. Look the exact address up on Croydon Council's conservation-area maps. On those maps a boundary line can run between two houses that were built as a pair, putting one inside and the other outside.
Planning applications show what the neighbours are actually doing. Between April 2024 and June 2026 the planning register logged 796 applications across Thornton Heath, 343 of them in the last twelve months. In a district made mostly of terraces, that is a lot of party walls getting a letter.
The ground is the one constraint that covers every house. The British Geological Survey rates every postcode in Thornton Heath at class 3 for shrink-swell clay, the kind of ground that swells when wet and shrinks when dry. That is not a reason to give up on the rear extension. It means foundations probably go deeper than a builder from a chalk or sand area would expect, and a structural engineer should look at the design early, especially near trees. The district once had gravel pits and market gardens. The clay is the part of the ground that keeps moving.
Day to day, most of the terraces and semis here sit outside the listed buildings and the conservation areas. That usually leaves rear extensions and loft conversions to the ordinary permitted development rules, with full planning permission needed only where a design goes past them. A house in one of the six conservation areas is a different case, and so is anything that changes the front.
Freedom to extend in Thornton Heath is middling. Planning is relaxed for most terraces, while the clay and the conservation areas at the edges mean some houses take more paperwork and deeper foundations.