Yes, you can extend a house in Brockley, but expect more scrutiny than in most of south London. The district has dozens of listed buildings. Three conservation areas touch it. The ground shifts with the weather everywhere here. Your neighbours file hundreds of planning applications a year, so getting approval is clearly possible.
Historic England lists 59 buildings in the area, two at Grade II* and 57 at Grade II, with no Grade I at all. Listing covers the inside of a building as well as the outside, so almost any change to a listed building needs listed building consent on top of any planning permission, and unauthorised work is a criminal matter. Brockley's heritage is mostly a streetful of good houses, without one great one at its centre.
Three conservation areas fall within the wider area around Brockley. In a conservation area, changes that would be routine elsewhere get looked at more closely, especially anything visible from the street, and demolition needs consent. The Victorian and Edwardian streets are also watched by the Brockley Society as well as the council, which is two more opinions than most back extensions attract. Ask Lewisham council early whether the specific address is inside a conservation area boundary and whether any extra restrictions apply there, because the answer can change from one side of a road to the other.
Neighbours are not put off. Planning records show 860 applications in the area between April 2024 and June 2026, 357 of them in the last twelve months. Plenty of people are trying to change their homes here, and the paperwork is part of everyday life.
The British Geological Survey puts the whole area in shrink-swell class 3, so the clay under Brockley swells when wet and shrinks when dry. For an extension this mostly means deeper foundations and a structural engineer's assessment, especially near trees. Brockley is known for its historic trees, and on ground like this, a large one close to a new foundation is the first thing an engineer will ask about.
Two of the area's listed buildings are on Historic England's Heritage at Risk register. Those are buildings in poor condition whose repair may qualify for grant funding and will certainly be watched closely. They make little difference to anyone extending an ordinary house a few streets away.
In practice, the starting point is what kind of home you own. Houses outside a conservation area keep the usual permitted development allowances for modest extensions. Flats, which make up about 55% of homes here, do not get the same allowances, and conservation area status cuts back what houses can do without permission. If you plan to extend a converted flat in a conservation area, assume you will need planning permission and work back from there.
Your freedom to extend here is middling. Plenty of projects do get built, but between the heritage rules, the conservation areas and the moving clay, almost every one needs consent and an engineer.