A rear extension is the most common home improvement project in England. The rules are better known than most — but the prior approval scheme for larger extensions trips people up constantly, and the 50% garden rule catches out almost everyone on smaller plots.
Single storey up to 4m (detached) or 3m (semi/terraced): no planning permission needed in most cases. Larger extensions up to 8m/6m: prior approval required. Two storey: planning permission required.
A single-storey rear extension is permitted development under Class A if it extends no more than 4 metres for a detached house or 3 metres for a semi-detached or terraced house beyond the original rear wall, does not exceed the height of the existing roof, is not higher than 4 metres at the eaves, does not extend beyond the side wall, does not cover more than 50% of the garden, and uses materials similar in appearance to the existing house.
Under the larger home extension scheme, a detached house can extend up to 8 metres and other houses up to 6 metres — but a prior approval application to the council is required. Neighbours are consulted for a minimum 21 days. The council can refuse on amenity grounds. Prior approval is not planning permission but it is a formal approval that can be refused and costs around £258.
Permitted development limits are measured against the original house as built — not the house as it currently stands. If previous owners already extended under permitted development, their extension counts. Your extension must not take the total beyond the Class A limit. The planning history shows previous extensions.
Extensions plus outbuildings must not together cover more than 50% of the original garden — on smaller plots, an existing outbuilding or garage can mean you have less extension allowance than you think. Extensions must also use materials of similar appearance to the existing house — a different material may require planning permission even if the size is within limits.
Standard searches check the public register. We go further — querying live portals, blocked legacy systems, pre-merger authority databases, committee PDF archives, Land Registry title constraints, and comparable decisions across your postcode cluster. What we retrieve determines what you know before you build, buy, or appeal.