Dormer vs Roof Window vs Hip-to-Gable: Which Loft Conversion Suits Your Home?
If your loft is full of old insulation, a cold-water tank and the faint smell of 1970s fibreglass, the question is rarely whether you can convert it. It's which of the three standard approaches gives you the best room for the money. A rear dormer, a roof window conversion and a hip-to-gable all create usable space, but they differ enormously in cost, headroom, planning risk and what your house looks like from the pavement. Here's how they line up.
The three options, in plain terms
- Roof window conversion. The roof stays exactly as it is. You insulate between and over the rafters, lay a floor, add a staircase and fit windows into the slope. Quickest, cheapest and the least disruptive to the house.
- Rear dormer. A box built out from the rear roof slope, with a vertical wall and windows. Adds genuine floor area and full-height headroom. This is the default choice for most Victorian terraces.
- Hip-to-gable. The sloping hip at the side of the roof is rebuilt as a vertical gable wall, reclaiming the dead triangle of space at the corner. Often paired with a rear dormer to give an L-shaped or full-width room.
None of these is "best" in the abstract. The right answer depends on your roof shape, your pitch and how much space you actually need.
Cost: what drives the bill
Roof window conversions usually come in lowest because the structure already exists. You're paying for insulation, plasterboard, a floor, a staircase, wiring and glazing — not for a new roof.
A dormer adds new construction on top of all that: a timber frame, cladding, roof covering, lead flashing and the labour to tie it neatly into the existing slope. The bigger the dormer and the more complex the junction with the main roof, the more it costs.
Hip-to-gable tends to sit at the top of the three. The hip has to come off, a new gable wall goes up, and steel is often needed to carry the load where the roof structure changes. Because it's frequently combined with a dormer, you're paying for two jobs at once.
Whatever you choose, these are the things that quietly push the price up:
- Structural work — steels, new floor joists, removing a chimney breast.
- Staircase position, and headroom over the flight.
- Plumbing for an en-suite, plus relocating a tank or boiler.
- Glazing specification, cladding and the external finish.
- Party wall agreements on terraces and semis, and building regulations fees.
Prices vary hugely by region and specification, so treat any "per square metre" figure you find online as a rough starting point only. Ask for itemised quotes that state what is and isn't included.
Headroom: the number that decides everything
Get into the loft with a tape measure before you fall in love with any design. Measure from the top of the ceiling joists to the underside of the ridge, note the roof pitch, and check the headroom at the eaves.
A roof window conversion only works well if the pitch is reasonably steep and there's a decent run of height down the middle of the roof. On a shallow pitch you end up with a room where you can only stand upright in a narrow strip, which is fine for a study and frustrating for a bedroom.
A dormer solves this by creating a vertical wall. Headroom is full height along the face of the dormer, tapering back into the slope. A hip-to-gable solves the other problem: it recovers the corner where the hip cuts in. On a semi or detached house with a hipped roof, that corner is often the difference between a cramped box room and a proper double.
As a rule, if you want a bedroom with an en-suite and a landing that doesn't feel like a tunnel, you need floor area as much as headroom.
Planning risk
In England, many loft conversions fall under permitted development, which means no full planning application. The conditions are well known: volume limits (broadly 40 cubic metres for a terraced house and 50 for detached and semi-detached), no dormer on the principal elevation that faces the highway, and materials that match the existing house.
Even when you're comfortably within those limits, apply for a Lawful Development Certificate. It's a modest piece of paperwork that proves the conversion was lawful, and it will save you an awkward conversation when you come to sell.
Ranked by risk, roof windows are the least contentious — a window in a rear slope rarely bothers anyone. A rear dormer is usually straightforward but worth checking, particularly if the property is a flat or maisonette, sits in a conservation area, or is covered by an Article 4 direction that removes permitted development rights. Hip-to-gable carries the most risk, because it changes the silhouette of the building and some councils take a stricter view of whether it's covered at all.
Rules differ in Scotland, Wales and Northern Ireland, and listed buildings are a separate conversation entirely. Check with your local planning authority before you commit any money, and take professional planning advice if the property is listed or in a sensitive area.
Kerb appeal and the view from the street
Roof windows leave the roofline untouched, which is why they're the natural pick for period terraces and conservation areas. From the front, nobody would know you'd done anything.
A rear dormer is invisible from the street on most terraces, but very visible from your garden and your neighbour's. Proportion matters: a dormer that runs the full width and sits too tall above the ridge line looks bolted on, and it will be the first thing a buyer notices.
Hip-to-gable is the boldest change. It alters the shape of the roof as seen from the side, and on a semi or an end-of-terrace that's visible from the road. Done well, it looks deliberate and adds presence. Done cheaply, with thin cladding and a mismatched tile, it drags down the whole house. Hipped roofs are often a defining feature of a street, so expect this to be the conversion most likely to attract comments during consultation.
How to choose, in order
- Measure the loft properly — ridge height, pitch, headroom at the eaves, chimney position.
- Decide what the room is for. A snug or home office can live with a slope; a main bedroom with an en-suite needs floor area.
- Look at what neighbours have done, and search your council's online planning register for approvals and refusals nearby.
- Confirm your permitted development position in writing with the local authority.
- Have a structural engineer look at the rafters and floor joists before you sign anything.
- Compare quotes on what's included, not on the bottom line alone.
Most homeowners end up with a rear dormer because it delivers the best balance of space and cost. If you have a hipped roof on a semi or detached house, hip-to-gable plus a dormer usually gives you the most room. And if the headroom is already there and the budget is tight, a well-insulated roof window conversion is a genuinely good room — just don't expect it to feel like a full storey.
Talk to a designer who has worked on your street or your house type; they'll know the local planners and the roof shapes. And take proper planning and structural advice before you commit, because correcting a wrong choice afterwards costs far more than getting it right at the start.
Photo: Gonzalo Facello / Pexels