Glossary
Rear extensions on a London terraced or semi-detached home
An extension at the back of the property. Most common London household extension.
An extension at the back of the property. Most common London household extension.
A rear extension projects from the back of the house. Under permitted development a single-storey rear extension may extend 3 m from the original rear wall on an attached house or 4 m on a detached one, up to 4 m high — or 6 m and 8 m respectively through the larger home extension prior approval route at £249.
Single storey: 3 m depth attached, 4 m detached, maximum height 4 m, and no more than 3 m high within 2 m of a boundary. Total ground coverage of all extensions and outbuildings must not exceed 50% of the curtilage excluding the original house. Materials must be similar in appearance.
Two storey: limited to 3 m depth, must be at least 7 m from the rear boundary, and the roof pitch must match the existing where practicable. Two-storey rear extensions are not permitted at all in conservation areas or on Article 2(3) land.
You notify the council with a plan and a written description, they consult adjoining owners for 21 days, and if no adjoining owner objects the extension may proceed. Where an objection is received the council decides on amenity grounds alone. The fee is £249 and the determination period is six weeks.
The extension must be completed and the council notified of completion. This route is unavailable in conservation areas.
For a full application, the 45-degree rule is the usual test: a line drawn at 45 degrees in plan and section from the centre of the neighbour's nearest ground-floor habitable window should not be crossed. Depth, height at the boundary and the effect on the neighbour's outlook do the rest.
A worked Harrow example of rear extension: with 29 conservation areas — the Harrow on the Hill Conservation Area and the Pinner Village Conservation Area among the best known — London Borough of Harrow (Harrow Council) sees rear extension constantly, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Where rear extension pushes a Harrow scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.
| Single storey, attached | 3 m (6 m via prior approval) |
|---|---|
| Single storey, detached | 4 m (8 m via prior approval) |
| Maximum height | 4 m, or 3 m within 2 m of a boundary |
| Two storey | 3 m depth, 7 m from rear boundary |
| Prior approval | £249, 6 weeks, 21-day consultation |
| Curtilage coverage limit | 50% |
3 m on an attached house, 4 m detached — or 6 m and 8 m through prior approval.
No. It is unavailable on Article 2(3) land.
A test of whether the extension crosses a 45-degree line from the neighbour's nearest habitable window.
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