Permitted development for extensions: what it actually covers

'It's permitted development, we won't need planning permission' is one of the most confidently wrong sentences I hear about extensions. The rights are real and genuinely useful, but they're a defined set of numeric and positional limits, not a general licence to build something reasonable — and the number of extensions built just outside those limits, on an honest mistake, is higher than it should be.

What PD actually is, and isn't

Permitted development is a general planning permission granted by national legislation rather than through an individual application, subject to specific limits on size, height, position and materials. It is not 'anything reasonable that fits in with the neighbourhood' — it's a defined set of rules that a proposal either does or doesn't satisfy, checkable against the drawings rather than a matter of judgement.

The headline limits for a single-storey rear extension

Depth limits differ by house type: up to 4m for a detached house under the larger home extension scheme (which requires a neighbour consultation process even though it's still PD), or 3m under the standard rules; up to 3m for other house types under the larger scheme, or fixed, smaller limits under the standard rules. Height limits apply at both eaves and ridge, and a single-storey rear extension generally can't exceed 4m in overall height.

Two-storey and side extensions have tighter rules

Side extensions are generally limited to a single storey and a proportion of the width of the original house. Two-storey rear extensions have their own separate, tighter depth limit, must not come within a set minimum distance of a boundary, and need a matching roof pitch where they exceed single storey — rules that catch out people assuming the single-storey limits simply scale up.

The situations where PD simply doesn't apply

Flats and maisonettes have no PD rights for extensions at all. An Article 4 direction can remove PD rights in a specific area regardless of the general national rules. A conservation area or AONB imposes additional restrictions, particularly on side extensions and roof alterations. And if the 'original house' — as it stood in 1948, or as built if later — has already been extended, previous additions count against the total allowance even if a different owner carried them out.

Materials and appearance conditions

PD extensions still carry conditions in most cases requiring materials to match the existing house, and specific rules on side elevation windows facing a boundary — obscure glazing, and non-opening below 1.7m in most cases. People miss these because 'permitted' reads as 'no conditions', when it actually means 'conditions set in advance rather than negotiated case by case'.

Getting certainty rather than assuming

A Lawful Development Certificate gives a definitive, checkable answer that a specific proposal genuinely falls within PD, rather than proceeding on an assumption that turns out wrong once the extension is built and a neighbour, or the council, disagrees. It's worth the modest fee and few weeks it takes, particularly on anything close to a numeric limit rather than comfortably within it.