Well if that is all you want it isn't too difficult. Admitted not large development sites like Loampit Vale or Catford Dog Track (or Bell Green which is really part of Forest Hill/Sydenham). Here's a start:
Council depot on Willow Way (in an employment zone, so hard to put in residential, but FH Soc have proved that it might be possible in terms of live/work)
Waldram Park Cresent (MOT garage and taxi rank)
Featherstone Lodge (which from the last I heard was empty, but there were some interesting ideas about using it for housing)
Brent Knoll School will be moving and I don't know what is planned for that site. Will it continue to be for education or for housing?
Across the road from Brent Knoll is a garage of some type. I have no idea what it is used for but suspect it could be better utilised
Behind Tyson Road has planning permission for 70 flats. I wouldn't put quite so many units there, I would prefer that if it must be developed it could be developed in the same way as Montgomery Mews
The Old Sorting Office has planning permission and I would like to see something done with that site, respecting the facade of the building
There are a number of sets of garages that are disused on private land, some of these would be better as housing (see attached photo which I have deliberately not identified as I have no idea about their utilisation or suitability as a site for housing, but it's from Google Maps). Most garages are no longer suitable for today's cars so they are used for storage (nothing wrong with that), or in some cases left to fall apart.

Is that enough sites for now? Each of these sites needs to be considered on its own merits and not all the development should be residential, we already have less employment in Lewisham than most other parts of London, so it is not really housing we are missing.

So there are plenty of small sites in Forest Hill for potential development, over and above a number of developments in recent years. But to meet the national housing shortage a larger project of house building is needed - new towns in areas that are currently have very low density levels but good transport connections. Forest Hill is not one of these locations but there are plenty around London and there are even more in the north of England. This is not the responsibility of amenity societies but of national and local government across the country.

What I don't think is the solution is bulldozing streets in Forest Hill to build modern high density low-medium rise development. If such a policy really was necessary I would advocate starting in Dulwich or Hampstead where density levels could be increased but smashing up fewer houses with massive gardens, or maybe golf could be played on 15 hole courses rather than 18 holes