What is the 50% rule for house extension?

The 50% is measured against the curtilage — the land that goes with your house — excluding the footprint of the original house itself. Add up everything built on it: past extensions, the shed, the garage, a garden office. If your planned extension takes the running total past half the land, it isn't permitted development and needs a planning application.

The phrase "original house" trips people up. It means the house as it was first built, or as it stood on 1 July 1948 if it's older than that. A 1990s conservatory or a previous owner's side extension doesn't reset the baseline — it consumes it.

This matters most on smaller plots. A terraced house with a modest garden and an existing rear addition may have far less allowance left than the owner assumes, while a detached house on a large plot rarely gets near the limit.

The 50% rule is only one of the permitted development tests — depth, height, boundary distances and materials all apply on top. Passing one test doesn't mean passing them all.

If you're close to the line, get formal confirmation before building. A Lawful Development Certificate proves your project qualifies as permitted development, which protects you when you sell.