What is the PD volume limit for loft conversions?

Under the General Permitted Development Order (GPDO), you can add up to 40m³ of additional roof space volume for terraced and semi-detached houses, or 50m³ for detached houses. This is cumulative — any previous roof extensions count towards the limit. Volume is measured externally.

How do I measure the volume addition?

The volume addition is the external volume of any enlargement beyond the original roof plane. For a dormer, this is typically Length × Width × Height measured externally. For a hip-to-gable conversion, it's the triangular volume added by extending the hip to a vertical gable wall.

Do Velux windows count towards the volume?

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.

What if I exceed the PD volume limit?

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 costs £258 in England (2025/26). Mansard conversions and front dormers also require planning permission regardless of volume.

Do previous extensions count?

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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Loft PD Volume Calculator — Permitted Development Roof Limits

Check if your loft conversion stays within Permitted Development volume limits: 40m³ for terraced/semi-detached, 50m³ for detached houses.

House Type

PD limit: 40 m³

Previous Roof Extensions

Volume of any existing roof additions

Dormer Length

External length of dormer

Dormer Width (Projection)

How far dormer projects from roof

Dormer Height

External height of dormer face

Hip-to-Gable Volume

Volume added by hip conversion

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 + Previous Extensions

Within PD = Total Volume <= Limit (40m³ or 50m³)

PD limits

Detached: 50 m³
Semi-detached: 40 m³
Terraced: 40 m³

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.