Permitted development, in plain terms
11 July 2026
Forty cubic metres on a terrace, fifty on a semi or detached, and a short list of things that void it entirely.
Most loft conversions in England do not need planning permission. They are built under permitted development, which grants an allowance of additional roof volume: 40 cubic metres for a terraced house and 50 for a semi-detached or detached one.
The allowance is cumulative across the life of the building, not per owner. A previous extension has already spent some of it, and the seller rarely mentions that. We check the planning history before we design, because a dormer drawn to an allowance that was used in 1998 is a dormer that gets refused.
The conditions that void it are short and absolute: nothing may extend beyond the plane of the existing roof slope on the principal elevation, nothing may sit higher than the existing ridge, and the materials must be similar in appearance. Conservation areas remove the allowance for side and front elevations entirely.
None of this is obscure. It is published, it is free to read, and it is still the most common reason a loft project stalls, because it is checked after the design rather than before it.
The measurement that decides everything
Not the floor area. The height from the new floor to the ridge, measured over where the stair will land.
Why drawings and build should be the same firm
Because the gap between a drawing and a buildable thing is where every stalled job lives, and split contracts mean nobody owns it.