Class B of the GPDO covers roof alterations, with a volume allowance that depends on house type.
Detached and semi-detached houses get up to 50m³ of additional roof space; terraced houses get up to 40m³.
The extension must not go beyond the plane of the existing roof slope on any elevation fronting a highway, and must not exceed the height of the existing roof.
Side-facing windows must be obscure-glazed and non-opening below 1.7m from the floor.
Verandas, balconies and raised platforms are not permitted development under Class B.
England only. Takes about a minute.
Last reviewed 2026-09-01. Guidance, not legal advice.