The five forms
- Rooflight conversion: rooms sit within the existing roof shape, with rooflights for light and ventilation. The existing slopes remain a major constraint on headroom.
- Dormer: an addition projects from a roof slope, creating space with a more upright wall and a window. Its position and dimensions determine how much useful headroom it adds.
- Hip-to-gable: a sloping end of a hipped roof is replaced with a vertical gable, enlarging that end of the roof space. It depends on the existing roof having a suitable hip. [1]
- L-shaped dormer: connected additions use the main roof and a rear or side roof, often called the outrigger or back addition. The government guidance expressly considers this joined arrangement; it is not a separate permitted-development class. [2]
- Mansard: the roof is reshaped with a steep lower face and an upper roof. Westminster's roof guidance distinguishes flat-topped and double-pitched forms. The design needs to suit the building and the surrounding roofscape. [3]
Compare the finished layouts, not just the labels
Ask for the stair, bed or desk, circulation and bathroom to be drawn in each option. Show finished headroom after the floor and roof construction has been allowed for. A scheme with a larger external volume does not necessarily give the most convenient room if the stair arrives in the wrong place. This is a design comparison, not a claim that one roof type is always preferable. [4] [5]
The structural design can also differ substantially. Altering rafters to form a dormer creates new support requirements; the loads from the enlarged roof and new floor must reach adequate supporting construction. None of the five names tells an engineer which beam sizes to use. [6] [7]
Which options can use permitted development?
For eligible houses, Class B covers qualifying roof enlargements and Class C covers qualifying roof alterations such as rooflights. Class B has cumulative volume limits of 40 cubic metres for a terrace and 50 cubic metres for other houses, together with separate height, location, materials and window conditions. Staying within a volume allowance is not enough by itself. [2]
A mansard is not automatically permitted or automatically prohibited because of its name. Test its actual geometry against the rules. Where a planning application is needed, the August 2026 NPPF's policy L2 supports effective use of airspace, including mansards, subject to the policy's safeguards and the other relevant policies. It is not a blanket planning permission. [2] [8]
Keep safety and budget in the comparison
Each habitable option needs a compliant floor, stair and fire strategy. Price the same scope across the options: roof work, internal alterations, services and finishes. A less extensive roof alteration can still need substantial work elsewhere in the house. [9] [10]