What is the 50% extension rule?
Permitted development requires that extensions and outbuildings together cover no more than 50 percent of the land around the original house. 'Original' means as built, or as it stood in 1948 — so previous owners' extensions count against your allowance, even if you did not build them.
The rule is set out in the government's permitted development guidance: all extensions and other buildings must not exceed 50 percent of the curtilage of the original house. Curtilage means the land around the house, excluding the footprint of the original building itself. Sheds, garden rooms and outbuildings count towards the total, not just extensions.
The word doing the work is 'original'. It means the house as it was first built, or as it stood on 1 July 1948 for anything older — which covers every Victorian house in Fulham. An extension added by a previous owner in 1985 counts against your 50 percent even though you had nothing to do with it. This catches people out regularly on Fulham terraces, where many houses have already been extended once.
For a typical Fulham terrace with a small rear garden, the 50 percent limit is often reached sooner than expected, particularly where a previous side-return infill or a garden room already exists. Checking it is straightforward — measure the original footprint and the garden, then total everything built since. If the sums are close, a Lawful Development Certificate is well worth having so the position is confirmed rather than assumed.
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