Yes, you can extend most homes in Forest Hill, and plenty of people are doing it, though the ground and two conservation areas set the terms. Only a small number of buildings are listed. Two pockets carry conservation-area rules. The whole hill sits on the kind of ground that cares about foundations. The planners are the easier half of the conversation; the soil is the one that never changes its mind.
Historic England lists 79 buildings in and around the area. None is Grade I, one is Grade II* and the other 78 are Grade II. Owning one means listed building consent for almost any change, inside or out, that affects its character, and that covers far more than extensions: windows, internal walls and fireplaces are all in scope. Outside those 79, the great majority of Forest Hill's homes carry no listing at all.
There are two conservation areas, Honor Oak Rise and The Gardens. Inside them, the council takes a closer interest in anything visible from the street, and works a house elsewhere could do without asking may need permission. Councils can also remove specific permitted development rights in a conservation area, so a buyer in either should ask Lewisham's planning department which rules apply to that street before planning anything. Two areas is a small share of the hill, and the line between them and everywhere else is where a loft conversion goes from routine to a negotiation.
Lewisham's planning register records 919 applications in the area between April 2024 and June 2026, and 376 of them in the last twelve months. That is a steady stream of kitchens pushed into gardens and lofts pushed into roofs. Forest Hill buyers clearly treat the house they bought as a first draft.
The British Geological Survey puts the area's shrink-swell hazard at class 3 at worst, and 100% of the area reaches that class. Shrink-swell ground swells when it is wet and shrinks when it dries, and a new extension sitting on shallower foundations than the house it is attached to can move differently from it. In practice that means deeper foundations, a structural engineer's view at the design stage, and care around trees near the new footings. None of it rules out an extension, but it sets the cost of the hole in the ground before anyone talks about the glass.
Historic England's Heritage at Risk register includes six listed buildings in and around the area. These are buildings in poor condition, whose repair may attract grant money and will certainly attract close scrutiny of any work done to them. For the owner of an ordinary terrace a few streets away, they change nothing.
Outside the listed buildings and the two conservation areas, many rear extensions and loft conversions on houses can go ahead under permitted development rights, subject to size limits and a prior check with the council in some cases. Flats are different: with flats making up most of the area's homes, a large share of buyers will need full planning permission for external changes, plus the freeholder's consent under the lease.
Forest Hill gives an average amount of freedom to extend. Most houses outside two conservation areas can grow, provided the owner pays an engineer to respect the ground before paying a builder to dig it.