Extension & Outbuilding Rules FAQs

The size limits, boundary distances and permitted development allowances that decide what you can build without a fight.

• What is the 50% rule for house extension?

  Under permitted development in England, extensions and outbuildings must not cover more than 50% of the land around the "original house" — the house as first built, or as it stood on 1 July 1948. Every existing extension and garden building counts toward that half, so previous owners' additions eat into your allowance.

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• How close can an extension be to a boundary?

  Under permitted development you can build a single-storey extension right up to your boundary, but if any part sits within 2 metres of a boundary, its eaves can be no higher than 3 metres. Two-storey extensions must stay at least 7 metres from the rear boundary. Build near or on the line and the Party Wall Act will usually also apply.

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• What is the 1 metre boundary rule?

  The "1 metre rule" usually refers to building regulations, not planning: an external wall within 1 metre of a boundary must be built to resist fire spread, with strict limits on unprotected openings like windows. People also use the phrase loosely for the practical advice to keep extensions about a metre off the boundary for gutters and maintenance access.

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• What size extension can you build under permitted development?

  For houses in England, a single-storey rear extension can extend 4 metres from the original rear wall for a detached house or 3 metres for any other house — rising to 8 metres and 6 metres with prior approval. Single-storey height is capped at 4 metres, and side and two-storey extensions have their own tighter limits.

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• What is the largest building you can build without planning permission?

  For a garden building in England, permitted development allows an outbuilding up to 4 metres tall with a dual-pitched roof (3 metres otherwise, 2.5 metres within 2 metres of a boundary), as long as all outbuildings and extensions together cover no more than half the land around the original house. Over 30 square metres of floor area, building regulations approval is usually needed too.

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• Can my neighbour build a shed against my fence?

  Usually yes — on their own land, a neighbour can place a shed right up to the boundary under permitted development, provided it's no taller than 2.5 metres within 2 metres of that boundary and passes the other outbuilding rules. What they can't do is attach anything to your fence, let gutters overhang your side, or cross the line.

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