Free tool
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.
1
Property
2
Dimensions
3
Previous
4
Verdict
Step 1 of 4
What type of property is it?
Your property type determines your Class B volume allowance.
Class B allowance: 40m³ for terraced and end-of-terrace houses; 50m³ for detached and semi-detached houses. Flats and maisonettes are not eligible for loft PD rights under Class B.
Terraced
40m³ limit
End-of-terrace
40m³ limit
Semi-detached
50m³ limit
Detached
50m³ limit
Flat / Maisonette
PD not available
Step 2 of 4
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.
m
m
m
Proposed volume
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Step 3 of 4
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.
Has any previous loft or roof extension been built?
m³
If you're not sure, we measure existing roof additions on survey and calculate the remaining allowance precisely.
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PD allowance (Class B limit)
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Volume already used by previous extensions
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Proposed addition volume
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Remaining PD allowance after proposed
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