Most loft conversions can be done under permitted development — but there are limits, and some common conversions fall outside them.
Under permitted development rights, you can convert your loft and add dormer windows without planning permission, provided the total volume added to the roof doesn't exceed 40 cubic metres for terraced houses or 50 cubic metres for detached and semi-detached houses. The roof shape and ridge height must not be altered.
Dormers or any roof alterations that face a highway always require planning permission. Any loft conversion on a listed building requires listed building consent (and often full planning permission). Properties in conservation areas may have permitted development rights removed. Hip-to-gable conversions usually require full permission.
Roof windows (Velux-style) that don't protrude beyond the plane of the roof are generally permitted development and don't count towards the volume limit. Dormers — which break through the roof slope — do count towards the limit and face more design scrutiny if they require planning permission.
The volume limits apply to the total addition above the original roof, not just the dormer itself. If a previous owner has already added a dormer, that volume may already count against your limit. Get your architect to calculate this before designing your scheme.
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