Rear dormer or hip-to-gable: which gives you more room?

This choice is usually settled by the shape of your roof rather than by preference, and you can work out most of the answer from the pavement outside your house.

Stand back and look at the side of the roof. If it slopes inwards towards the ridge, like the end of a tent, you have a hipped roof. If it goes straight up as a flat triangle of brickwork, you have a gable.

That one detail decides which of these two is even available to you.

What each one does

A rear dormer builds a box out from the back slope of the roof. The sloping ceiling that stopped you standing up becomes a vertical wall and a flat ceiling, so the floor underneath turns into usable room. The front of the house does not change.

A hip-to-gable rebuilds a sloping side of the roof as a straight gable wall, taking the roof out to the edge of the house. It does not add much headroom on its own. What it adds is length: the loft runs the full depth of the house instead of dying away where the hip starts sloping in.

If your roof is already gabled

Then hip-to-gable is not a decision you have to make. There is no hip to convert, and a rear dormer is the way to get standing height. This is the usual situation on a mid-terrace.

The question becomes how wide to make the dormer, not which type to build. Within the volume the rules allow, wider is almost always better value, because the scaffold, the steels and the structure are already paid for.

If your roof is hipped

This is common on semi-detached houses, end-terraces and detached houses, and it is where the choice is real.

A dormer alone on a hipped roof leaves the sloping end unusable. You get a good room at the back and then a wedge of space at the side that is too low to stand in and too awkward to furnish. It is often just enough space to be annoying.

Hip-to-gable squares that end off. Paired with a rear dormer, it usually turns one cramped room into a bedroom plus a shower room, which is the combination most people are actually after.

That is why our loft conversions page lists hip-to-gable with rear dormer as one option rather than two. On a hipped roof they work as a pair.

Cost, time and disruption

A rear dormer is typically quoted between £45k and £75k and takes six to eight weeks on site. A hip-to-gable with a rear dormer is typically £55k to £90k and takes eight to ten weeks.

The difference is mostly structure and brickwork. Rebuilding a hip as a gable means taking a section of roof apart, building a new masonry wall up to the new ridge line, and reroofing where the two meet. It is more work than a dormer alone and it is visible from the street, so the detailing matters.

Both are usually permitted development, within the same volume limits, as long as nothing goes above the ridge and the front slope is untouched. Our guide on planning permission for a loft conversion covers the conditions.

What decides it in practice

Three things, in this order.

The roof you have. A hip is the only reason to consider hip-to-gable at all.

What the space is for. One bedroom rarely justifies the extra work. A bedroom with a shower room, or a room you want to put a double bed and a wardrobe in, usually does.

Your volume allowance. If a previous owner already extended the roof, there may not be enough left for both. That is a measurement, not a guess, and it is one of the things we check at the survey.

Measure from the top of the ceiling joists to the underside of the ridge before you plan anything. If that figure is around 2.2m or more you have options worth comparing. If it is less, the conversation changes, and a dormer or mansard is what creates the height rather than reveals it.

If you would like someone to measure it and tell you which of these your roof actually suits, book a survey.