What size extension can you build under permitted development?

The standard allowances for a single-storey rear extension are 4 metres of depth for detached houses and 3 metres for semi-detached and terraced homes, measured from the original rear wall. Maximum height is 4 metres, dropping to 3 metres at the eaves if you're within 2 metres of a boundary.

The "larger home extension" scheme roughly doubles the depth — up to 8 metres detached, 6 metres otherwise — but only via prior approval: you notify the council, adjoining neighbours are consulted for 21 days, and the council has 42 days to decide whether the impact on neighbours is acceptable.

Side extensions under permitted development must be single storey, no wider than half the width of the original house, and within the same height limits. Two-storey extensions can project only 3 metres beyond the rear wall and must stay 7 metres off the rear boundary.

All of this sits under the umbrella conditions: no more than 50% of the land around the original house covered, materials of similar appearance, and nothing forward of the principal elevation. On designated land — conservation areas, national parks — the allowances shrink or disappear: rear extensions are cut to 3 metres (4 metres detached), side extensions and cladding aren't permitted at all.

Two important caveats. These rights apply to houses, not flats or maisonettes, which have no permitted development rights for extensions. And an Article 4 direction can strip rights from any street — which is exactly the kind of constraint worth confirming for your specific address before you design anything.