How big an extension can I build without planning permission?

Under permitted development (PD) rights in England you can often extend without a planning application, within limits. For a single-storey rear extension, the standard allowance is up to 4m deep on a detached house or 3m on a semi or terrace, with a maximum height of 4m (lower if you're within 2m of a boundary). Under the “Larger Home Extension” route you can go up to 8m deep on a detached or 6m on a semi or terrace, but that needs prior approval — a 21-day neighbour consultation run through your council. Single-storey side extensions can be up to half the width of the original house and 4m high.

The big caveats: the limits apply to the house as originally built, so previous extensions may already have used up your allowance; and PD rights don't apply to flats or listed buildings, and are often removed in conservation areas, Article 4 zones, National Parks and AONBs. Rules also differ in Wales, Scotland and Northern Ireland.

It's not only single-storey work, either. Two-storey rear extensions can fall under permitted development up to 3m deep on many houses, provided they stay at least 7m from the rear boundary, roughly match the existing roof pitch and don't project past the principal elevation. Whatever the size, remember two things permitted development never removes: you still need Building Regulations approval for the structure, and if you build on or right up to a shared boundary the Party Wall Act can require formal notice to your neighbour weeks before work starts.

Before building anything, check your specific property on the Planning Portal and with your council — a lawful development certificate is cheap insurance. Our extension planning permission guide covers all the size limits, and the extension cost guide helps you budget.