You can extend a house in Blackheath, but you will be asking permission more often than in most of London. Hundreds of buildings are listed. Parts of the area are conservation areas. The clay underneath has its own opinions too. Your neighbours are still applying in large numbers, which tells you it can be done with patience.
Historic England lists 375 buildings across the Blackheath area: 19 at Grade I, 27 at Grade II* and 329 at Grade II. Listing covers the whole building, inside and out, along with things fixed to it and often the walls around it. Almost any change that affects its character needs listed building consent on top of planning permission, and doing the work without it is a criminal offence. Most of the list is Grade II, but most of the list is still the list.
There are two conservation areas in the area. Inside them, the council controls demolition, works to trees, and changes to the look of a street, and some of the alterations a householder elsewhere could make without asking can be withdrawn by local direction. Greenwich and Lewisham councils both publish their conservation-area maps, and since Blackheath straddles the two boroughs, it is worth checking which council's map covers the house and whether its road is inside a boundary.
The area also has 8 scheduled monuments, the strictest tier of protection, which mainly matters for anyone whose land runs up against one.
Planning portal records show 1,785 applications across the area from April 2024 to June 2026, and 818 of them in the last twelve months. Whatever the rules, people here are clearly still trying to add a room. Many of those applications will be for small things, and the number counts how many times people applied, which is a separate matter from how many times the council said yes.
The British Geological Survey puts the shrink-swell hazard at class 3 across all of the area. That is clay ground that swells when wet and shrinks when dry, so it can move the foundations of anything built on it, and trees close by make it worse. For an extension this means deeper foundations than a buyer might budget for, and a structural engineer's view before the design is fixed. The heath itself was dug for gravel, sand and chalk through the eighteenth and early nineteenth centuries, and in 1945 the pits were filled with bomb rubble. The common has been dug up and rebuilt more thoroughly than most of the houses facing it will ever be allowed to be.
Ten of the area's listed buildings are on Historic England's Heritage at Risk register. These are buildings in poor condition, whose repair may attract grant money and will certainly attract scrutiny. For the buyer of an ordinary house down the road they don't change much, though they do show that the conservation officers here have plenty of buildings to keep an eye on.
In practice, a house outside the conservation areas and off the list can usually use the national permitted development rights for modest rear extensions and loft work, subject to the usual size limits and checks. A listed house, or one in a conservation area, can lose much of that freedom, and something as small as replacement windows can become an application. A flat, which most homes here are, has no permitted development rights at all.
Blackheath is a hard place to extend a home. The planning rules are tight, the clay is fussy, and a Georgian facade can turn a simple rear kitchen into a year of letters.