Loft Conversion Permitted Development Volume Calculator

Terraced houses get 40 cubic metres of additional roof volume. Semi-detached and detached houses get 50 cubic metres. The volume is measured against the original roofline — any previous loft extension on the property counts against your allowance, even if you didn't build it. The standard rear dormer on a 4-bed terraced house typically lands around 35 to 42 cubic metres, so terraces often need careful design to stay under PD. Mansards add the most volume and usually need planning permission as a result.

Roof extensions are classed as 'designated land' work in conservation areas and have PD rights removed under Article 4 directions or Article 2(3) of the General Permitted Development Order. The reasoning is that loft conversions are visible from the street and change the roofline, which conservation areas are specifically protecting. If you're in a conservation area, you need full planning permission — and design will need to be sympathetic to local roofscape. The good news: most well-designed mansards in London conservation areas do get approved, they just take 8-12 weeks and pre-app dialogue with the planning officer.

A rear dormer is a box-shape that pushes out from the rear roof slope — adds head height across the back of the loft. Hip-to-gable converts the sloped side of a hipped roof into a vertical gable wall — adds floor area at the side, common on semi-detached houses. L-shaped is a dormer plus hip-to-gable, doubling up the gains and usually needed for full master-suite layouts. Mansard is the most dramatic — the entire rear roof slope is rebuilt to near-vertical, then a flat top, giving full ceiling height across the whole footprint. Mansards add the most space and value but cost 60 to 80 percent more than a dormer and almost always need planning permission.

Partially — it flags conservation areas which is the most common Article 4 scenario. But individual streets can have property-specific Article 4 directions that remove PD rights even outside conservation areas. Always check your council's planning portal or use the Planning Permission Risk Checker for postcode-level restrictions before committing to a design. I check the full constraints list (Article 4, conservation, listed buildings within 50m, TPOs) on every loft before quoting.