Permitted Development Limit Checker

Permitted development lets you build a single-storey rear extension without a planning application, provided every size and position condition is met at once. On a semi-detached or terraced house the standard rear depth limit is 3m from the original house rear wall; on a detached house it is 4m. This checker tests each condition against your dimensions and tells you which you pass, which you fail, and what your options are if you go over.

The conditions are cumulative, not a menu. Pass the depth limit but fail the eaves rule and the extension still needs planning permission. Enter your property type and measurements above and the tool runs every check in one pass.

The limits this checker tests

Four measurements decide whether a rear extension stays inside permitted development:

• Depth from the original rear wall: 3m for a semi-detached or terraced house, 4m for a detached house.
• Overall height to the highest point of the roof: capped at 4m, measured from the highest adjacent ground level.
• Eaves height where the roof meets the wall: if any part of the extension sits within 2m of a boundary, the eaves cannot exceed 3m.
• Curtilage coverage: all extensions and outbuildings together must not cover more than 50% of the land around the original house.

Two conditions this tool cannot measure still apply. The similar materials condition requires the extension to look like the existing house, and exclusions such as a listed building, a conservation area, or an Article 4 direction can remove permitted development rights entirely. A flat or maisonette has no permitted development rights for extensions at all.

If you exceed the depth limit

Going over the standard depth does not automatically mean a full planning application. The Larger Home Extension scheme allows a deeper single-storey rear extension, up to 6m on a semi-detached or terraced house and up to 8m on a detached house, through a prior approval process. You notify the council, adjoining neighbours are consulted, and if no valid objection stands the extension proceeds. The application fee is £249. This route is not available in a conservation area or on other designated land.

A worked example

Take a semi-detached house with a proposed rear extension that projects 3.5m from the original rear wall, reaches 3.4m at the ridge, and has 2.6m eaves sitting 1m from the side boundary. Estimated site coverage after the work is well under half the plot.

The overall height clears the 4m limit and the eaves clear the 3m boundary cap, so those pass. Coverage is inside the 50% curtilage limit. The depth is the problem: 3.5m is beyond the 3m standard limit for a semi-detached house. Because that is the only failed check and the depth is still within 6m, the extension qualifies for prior approval under the Larger Home Extension scheme rather than a full application.

Whatever the result, permitted development removes only the planning application. Building control approval is still needed, and a Lawful Development Certificate (£274) is worth having as written proof the extension was lawful, especially when you come to sell.