Permitted development for extensions and outbuildings is only available while buildings other than the original house cover no more than half the garden.
"Curtilage" means the land around the house within its boundary. "Original house" means the house as built, or as it stood on 1 July 1948 if built before then.
Every extension and outbuilding built under permitted development counts toward the 50% limit - including ones built by previous owners.
Once you're at or over 50%, further permitted development building work needs full planning permission.
England only. Takes about a minute.
Last reviewed 2026-09-01. Guidance, not legal advice.