Loft Volume Calculator
Check whether your loft conversion falls within the permitted development volume limits before you commit to a design. This calculator estimates the additional roof volume for rear dormers, hip-to-gable conversions, and combined works under Class B of the GPDO 2015.
The Class B volume rule
This limit is cumulative — it includes any previous loft or roof additions since the original house was built (or since 1 July 1948). Volume is measured using external dimensions, not internal floor area.
Conservation areas, AONBs and National Parks have additional restrictions on dormers visible from the highway.
Look up your property (optional)
Enter your address and we'll auto-detect your house type. Or skip this and select manually below.
Calculate your volume
Select your house and conversion type, then enter the external dimensions of the proposed roof addition. For combined works (e.g. hip-to-gable plus rear dormer), enter two volumes directly.
Volume of any existing loft or roof additions since the house was originally built. Enter 0 if none. If a previous owner added a small dormer, include its volume here.
Other rules beyond volume
- Ridge height: The conversion must not exceed the height of the existing ridge. You cannot raise the ridge line.
- Front dormers: Dormers on any roof slope facing a highway are not permitted development. Front dormers almost always need planning permission.
- Side windows: Any side-facing window must be obscure glazed and non-opening (or top-hung opening above 1.7m).
- Materials: The dormer must use materials similar in appearance to the existing house.
- Party wall: If a steel beam bears into a shared wall, a Section 2 notice is required at least 2 months before work starts.
- Building regulations: Always required for a loft conversion, covering structural work, fire safety, stairs, insulation, and sound insulation.
- This is a volume estimate only — get an architect to confirm precise external dimensions before proceeding.