What is the difference between a dormer, mansard and hip-to-gable?
A dormer adds a flat-roofed box projecting from an existing roof slope. A mansard rebuilds the roof at a near-vertical angle for maximum space. A hip-to-gable squares off a sloping side roof on an end-terrace or semi. Cost rises in that order, as does the likelihood of needing planning permission.
A dormer is an addition to the existing roof: the slope is opened up and a box with vertical walls and a flat roof is built out from it. It is the cheapest way to create full-height space, at £55,000 to £85,000 in Fulham, and a rear dormer commonly falls within permitted development. It is the default for a mid-terrace.
A mansard replaces the roof structure with a steeply pitched form, typically around 70 degrees, with a small flat roof at the top. It maximises floor area and generally looks more architecturally coherent on a Victorian terrace than a large dormer. It costs £80,000 to £110,000 and almost always needs planning permission — in some conservation areas it is the form planners prefer, precisely because it reads as traditional.
A hip-to-gable applies only where a roof slopes on a side elevation, so end-of-terrace, semi-detached and detached houses. The sloping hip is rebuilt as a vertical gable wall, creating volume along the full length of the roof. It is nearly always combined with a rear dormer, and the two together typically produce the largest usable loft of any option on a suitable house.
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