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Loft PD Volume Calculator — Permitted Development Roof Limits
Check if your loft conversion stays within Permitted Development volume limits: 40m³ for terrace houses, 50m³ for semi-detached and detached houses (GPDO 2015 Class B.1(d)).
Volume of any existing roof additions
External length of dormer
How far dormer projects from roof
External height of dormer face
Volume added by hip conversion
If raising the ridge line
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How We Calculate This
This calculator checks whether your proposed loft conversion stays within Permitted Development volume limits.
The formula
Total Volume = Dormer Volume + Hip-to-Gable + Ridge Raise + Previous Extensions
Within PD = Total Volume <= Limit (40m³ or 50m³)
The dormer volume is approximated as Length × Width × Height — the bounding box of a box (flat-roof) dormer. The actual legal test is the increase in the ‘cubic content of the resulting roof space’ over the original roof space (GPDO 2015 Schedule 2 Part 1 Class B.1(d), read with the B.3 interpretation). For pitched-roof or irregular dormers this bounding-box figure overstates the added cubic content, so the tool errs on the cautious side; confirm the actual content with an architect or measured survey.
PD limits (GPDO 2015 Class B.1(d))
- Detached: 50 m³
- Semi-detached: 50 m³
- Terrace: 40 m³ (only terrace houses are limited to 40m³; ‘50 cubic metres in any other case’)
The allowance is cumulative: previous roof enlargements over the life of the property count towards it (Class B.3).
Frequently Asked Questions
Under the General Permitted Development Order 2015 Schedule 2 Part 1 Class B.1(d), you can add up to 40m³ of additional roof space for terrace houses, or 50m³ for any other case (including detached AND semi-detached). This is cumulative — any previous roof extensions count towards the limit. Volume is measured externally.
The legal test (GPDO 2015 Schedule 2 Part 1 Class B.1(d), read with the B.3 interpretation) is the increase in the cubic content of the resulting roof space over the original roof space, measured externally. This calculator approximates a box (flat-roof) dormer as Length × Width × Height; that bounding-box figure overstates the added cubic content for pitched-roof, sloped-face or irregular dormers, so it errs on the cautious side. For a hip-to-gable conversion it's the roughly triangular volume added by extending the hip to a vertical gable wall. For pitched or irregular dormers, confirm the actual cubic content with an architect or a measured survey before relying on PD rights.
No. Rooflight windows that sit flush with (or within) the existing roof plane do not add volume and therefore do not count towards the PD limit. Only dormers and other projections beyond the existing roof plane are measured.
If your proposed loft conversion exceeds the PD volume limit, you will need full planning permission from your local authority. This typically takes 8 weeks and the householder planning application fee in England is approximately £548 from 1 April 2026 (the fee is CPI-indexed annually under the Levelling-up and Regeneration Act 2023 — confirm the current figure on the Planning Portal fee schedule). Mansard conversions and front dormers also require planning permission regardless of volume.
Yes. The PD volume allowance is cumulative over the life of the property. Any previous roof enlargements — even those done by a previous owner — count towards the limit. Check your property's planning history or look for evidence of previous roof alterations.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.