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.

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.