Loft conversion volume calculator
Check a dormer and hip to gable against the 40 or 50 m³ limit.
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Loft conversions can usually be built as permitted development if the added roof volume stays under 40 m³ for a terraced house or 50 m³ for a semi detached or detached house.
Enter the size of your rear dormer and any hip to gable extension and the calculator adds up the volume using the standard geometry, then tells you how much allowance you have left.
Questions people ask
How is dormer volume calculated?
A box dormer is roughly a triangular prism, so the volume is half the width times the height times the depth.
Do previous roof extensions count?
Yes. Any earlier roof additions count towards the 40 m³ or 50 m³ allowance, even if a previous owner built them.
Can I build a dormer in a conservation area?
Not under permitted development. Roof extensions in a conservation area need planning permission.
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