Why your South London borough sets the extension
The permitted-development rules for extensions are national, but the answer you get in South London is local. 7 councils — Croydon, Kingston upon Thames, Lambeth, Merton, Richmond upon Thames, Sutton and Wandsworth — read the same rules against their own local plans, their own rear-garden and daylight policies, and their own street character. The 5-point spread between Lambeth and Croydon in the table below is that difference made visible.
The stock decides the shape. South London is built largely from Victorian and Edwardian terraces, Victorian and Edwardian terraces in Surbiton, Georgian and Victorian terraces, Victorian and Edwardian villas and terraces, interwar semi-detached houses and Victorian terraces, and the extension that works follows from it: a side-return infill where terraces have a narrow alley beside the back addition, a straight single-storey rear where there is a full-width garden, a wraparound where both exist. Depth is where applications fail — the single-storey rear limits (3 metres on a terrace or semi, 4 on a detached house, or 6 and 8 under the larger-home prior-approval route) are counted from the original rear wall, not the one standing today.
Then the designations. South London holds conservation areas including Addiscombe (Canning and Clyde Roads), Upper Norwood Triangle and Park Hill, where extension permitted-development rights are curtailed and materials are assessed, and 7 of its 7 boroughs add Article 4 directions street by street. Neither follows a borough boundary, which is why we check the address itself before drawing, and why the regional average is context rather than a prediction.
