Start with permitted development
For a house (not a flat), a single-storey rear extension can usually be built without planning permission if it extends no more than 3 metres from the original rear wall on a terraced or semi-detached house, or 4 metres on a detached house, and is no higher than 4 metres. Under the larger home extension scheme those limits rise to 6 and 8 metres respectively, but you must notify the council first and neighbours get a say.
Where permitted development stops
Conservation areas, listed buildings, flats and houses where rights have been removed by an Article 4 direction all need a full householder application. In Merton, Wimbledon Village and Merton Park are the big ones; in Sutton, Cheam and Carshalton villages; in Lewisham, Blackheath and Forest Hill.
The 45-degree rule
Most South London councils apply a version of it. From the centre of your neighbour's nearest ground-floor habitable-room window, a line is drawn at 45 degrees; if your extension crosses it, the council will expect a daylight justification or a reduced depth. Side-return extensions on terraces are the classic case.
Timescales
Householder applications have an 8-week statutory period. Add two to four weeks for design and survey, and a week for the council to validate the application. Pre-application advice, where the site is sensitive, adds a few weeks but improves the odds.
This guide is general information, not advice on your property. Rules change and each council interprets them differently — we check every project against the current position.