Class B
Loft conversions: the permitted development rules that catch people out
Anatolie Chiriac · Graduate Architectural Technologist
Last reviewed August 2026. Relates to England. General guidance, not advice on a specific property.
A loft conversion with a rear dormer is usually permitted development under Class B of the General Permitted Development Order. It is also the class where schemes most often come unstuck, because several of the conditions are easy to miss on a drawing and impossible to fix once built.
Volume is the headline limit
The additional roof space created must not exceed 40 cubic metres for a terraced house, or 50 cubic metres for a detached or semi-detached house. Crucially this is cumulative: any previous roof enlargement, including one built by a previous owner decades ago, counts against the allowance. A 38 cubic metre dormer on a house that already gained 6 cubic metres in the 1980s is not permitted development.
The 0.2m eaves setback
The edge of the enlargement closest to the eaves must, so far as is practicable, sit at least 0.2m back from the eaves, measured along the roof slope — not horizontally. The original eaves must also be maintained or reinstated. Removing the eaves entirely so the dormer wall runs flush with the wall below takes the scheme outside Class B, and appeal inspectors have been consistent about this.
There is an exception for hip-to-gable enlargements and for an enlargement joining the original roof to the roof of a rear or side extension. But that exception protects the joint itself — every other edge still owes the setback unless there is a practical or structural reason it cannot be achieved.
Windows in side elevations
Any window inserted in a side elevation of the roof must be obscure-glazed, and non-opening unless the opening part is more than 1.7m above the floor of the room it serves. Both conditions, not either.
Other conditions
- No part of the enlargement may extend beyond the plane of the existing roof slope that fronts a highway.
- The enlargement must not exceed the highest part of the existing roof.
- Materials must be of similar appearance to the existing house.
- No verandas, balconies or raised platforms.
- Class B does not apply at all on Article 2(3) land — conservation areas, national parks, AONBs and World Heritage Sites.
Get the volume calculation right first
Almost every dispute starts with the volume figure. Before anything is drawn in detail, the existing roof volume and any historic additions should be established from measured information, not assumed.
This article covers the position in England and is general guidance rather than advice on a specific property.
Whether these rules let you build what you have in mind depends on measured dimensions and your property’s planning history — neither of which can be settled from an article. If you want a view on your own house, send us the details.
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