Start with your roof shape
The choice between a dormer and a hip-to-gable conversion is set mostly by the roof you already have. A gable roof has vertical triangular end walls. A hipped roof slopes down on the ends as well as the front and back, which is common on 1930s semis and many detached houses. Hips eat into loft space, and that is exactly the problem a hip-to-gable conversion solves.
What a dormer conversion is
A dormer is a box-shaped extension built out from the slope of the roof, usually at the back, with vertical walls and a flat or shallow roof. It turns a sloping ceiling into full headroom across most of the floor. A full-width rear dormer is the most common way to get a usable bedroom and bathroom in a loft.
Dormers work on almost any roof shape with enough height at the ridge, including gable and terraced roofs.
What a hip-to-gable conversion is
A hip-to-gable conversion straightens the sloping hip at the end of the roof into a vertical gable wall, extending the ridge out to meet it. That brings the staircase and floor out into the space that used to be under the hip. It is only relevant on houses with a hipped end, and on a semi it is the side of the roof away from the neighbour.
Most hip-to-gable conversions are combined with a rear dormer, because on their own they add length but not much headroom at the back.
Space and headroom compared
A rear dormer adds headroom across the back of the loft. A hip-to-gable adds floor area at the side and makes room for a stair coming up in the right place. Together they give the largest loft. On a hipped semi, a dormer alone can leave a cramped space with the stair squeezed under the slope.
Structure and brickwork
Both need new floor joists, steel beams in most cases, and alterations to the roof structure, all designed by an engineer. A hip-to-gable also means building a new gable wall in brick or block up from the existing side wall, matched to the house. That is masonry work that needs a good brick match on show, which is where an experienced bricklaying team earns its keep.
Planning
Many rear dormers and hip-to-gable conversions fall under permitted development, within volume limits and provided materials are similar to the house and nothing faces the road with a dormer. Conservation areas, flats and houses already extended may need an application. Building Regulations apply to all loft conversions.
Our loft work
Loft conversions are listed under our conversions service, and we have more than 26 years' building experience, NVQ Level 2 brickwork, FMB membership and full insurance, working from Bidwell, Dunstable. For the step-by-step process, see the loft conversion guide; if you want a structure-only scope, see the shell loft conversion guide.
Other options to know about
Dormers and hip-to-gable conversions are the most common, but not the only routes. A rooflight or Velux conversion adds windows in the existing slope with no change to the roof shape; it is the simplest option but only works where there is already good headroom. A mansard conversion rebuilds the rear slope almost vertical, with a flat top, giving the most space of all but usually needing planning permission. On some houses an L-shaped dormer wraps over a rear wing.
Whichever type you choose, the key checks are the same: headroom at the ridge, where the stairs will land on the floor below, and how the new rooms will escape in a fire. Settle those three and the choice of conversion type often becomes obvious.
It is also worth asking the neighbours early, as a party wall notice is often needed for loft work on a semi or terrace.
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Questions people ask
Which gives more space?
A hip-to-gable combined with a rear dormer gives the most. On a gable-ended house, a rear dormer alone is usually enough.
Can a terraced house have a hip-to-gable?
Usually not, because mid-terraces rarely have a hipped end. Dormers are the normal route.
Do both need Building Regulations?
Yes. All loft conversions need Building Regulations approval covering structure, fire safety and insulation.