Is My Loft Conversion Within Permitted Development Limits?
Under Class B of the GPDO, loft additions up to 40m³ (terraced) or 50m³ (detached / semi-detached) can be built without planning permission — if every other Class B rule is also met.
How is permitted development roof volume measured?
Against the roof as originally built, not as it stands today. Class B allows 40 cubic metres on a terrace and 50 on a semi or detached house, and any earlier dormer counts against the allowance even if a previous owner built it.
| Class B rule | Terrace | Semi or detached |
|---|---|---|
| Added roof volume allowed | 40 m³ | 50 m³ |
| Measured against | the roof as originally built | the roof as originally built |
| 'Original' means | as first built, or as at 1 July 1948 | as first built, or as at 1 July 1948 |
| Beyond the front roof plane | not permitted | not permitted |
| Above the existing ridge | not permitted | not permitted |
| Set back from the eaves | at least 200mm | at least 200mm |
| Inside a conservation area | Class B does not apply | Class B does not apply |
Source: the Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class B, as amended, read at 1 September 2026. The allowance is cumulative, which is why a house with an existing dormer often has far less headroom than the calculator's headline figure suggests until the earlier volume is deducted.
What type of property is it?
Your property type determines your Class B volume allowance.
Proposed loft addition dimensions
Enter the approximate dimensions of the volume you want to add (e.g. a rear dormer enclosure). For L-shaped or hip-to-gable schemes, measure each segment separately and sum the volumes.
Has any previous loft or roof addition been made?
Class B allowance is cumulative — any previous loft/roof additions (including by previous owners, since 1948) count against your 40m³/50m³ limit.
If you're not sure, we measure existing roof additions on survey and calculate the remaining allowance precisely.