Dormer or roof window: which is better?

A roof window lies flat in the slope of the roof. It lets in daylight and a view of the sky, but it adds no volume at all. A sloping ceiling stays a sloping ceiling, and a bed or wardrobe still has to sit where the head clears the rafters.

A dormer is a small box built out from the slope, with its own vertical window and usually capped by a roof of its own, flat or pitched. The vertical walls (cheeks) and the extra roof create real standing height and floor space where there was only a sloping void before. That is why dormers are the usual route for a bedroom or bathroom in a loft where the pitch is shallow.

Neither is better in the abstract. The question is whether your loft lacks light, or lacks room. Plenty of lofts only need the first, and a roof window handles that perfectly well.

A roof window is one opening. MyBuilder's roof window guide, updated in August 2026, puts typical installation at £350 to £600 for standard jobs, with a standard window at about £305 and scaffolding around £500. A dormer is a small building project: new framing, a new roof, cladding, insulation, windows and internal finishing. National guides price it as a build, not a fitting, so it needs a survey and a written quote before any figure means anything.

Disruption follows the same pattern. A single roof window can be fitted with the roof open for a short spell. A dormer leaves a large part of the roof open while the structure goes in, which makes a dry spell of weather and good temporary protection part of the plan.

This is where the two diverge sharply. Roof windows fall under Class C of the permitted development rights for houses. They can be installed without a planning application so long as they do not project more than 0.15 metres from the roof plane or rise above the original ridge, and side-slope windows are obscure-glazed. These rights can still apply in a conservation area or National Landscape. Our page on roof window planning has the checks.

Dormers come under Class B, which has tighter limits. Extra roof space must stay within 40 cubic metres for a terraced house or 50 cubic metres for any other house. The dormer cannot extend beyond the plane of a roof slope on the principal elevation that fronts a highway, and its top has to stay below the existing ridge. Crucially, Class B does not apply on article 2(3) land, which takes in conservation areas and National Landscapes such as the North Wessex Downs. There, a dormer needs a planning application.

Both options need Building Regulations approval when they alter the structure. Before you commit to either, put the question to your own council's planners (Swindon Borough Council, for addresses in the borough). A short conversation early costs nothing and can save a redesign. If the house is on the statutory list, consent for the building itself applies to roof windows as much as to dormers.

Per square metre of glass, a roof window gathers much more daylight than a wall window, since it looks straight at the sky. That is a gift in a dim landing or study and a drawback in a south-facing bedroom in July. Our page on why a skylight makes a room too hot explains the glazing and blind choices that control it.

A dormer window is vertical, so it brings in less light per square foot of glass but tolerates summer sun better, and it opens like an ordinary window. Dormers also create extra roof and wall area, which has to be insulated to the current standard; the extra surface is one reason their running costs are not trivial.

If the loft is awkward for either, a sun tunnel brings light to a landing or small room without any structural work at all.

Dormers are conventional windows at wall height, so the security question is the familiar one for any first-floor window. Roof windows sit on the roof where they are harder to reach, and laminated glass and decent locks help; see whether roof windows are secure.

On maintenance, a dormer is more to look after. It has a roof, cheeks, flashings and often a lead-lined gutter or valley, each a place for a leak to begin. A roof window has one frame and one flashing set. Our roof window maintenance page covers the routine checks.

Start with the purpose. If the loft will be a bedroom and the headroom is poor, go for a dormer if the planning position allows it. If the loft will be a study, a landing or a playroom, or if the house stands in a conservation area, roof windows are usually the sensible path.

Plenty of homes end up with both: a dormer for the main room and roof windows for the stairwell or bathroom. We survey the roof, send you photographs of your own roof, and give you a written quote so the choice rests on facts about your house. We would rather recommend the smaller job if it solves the problem, and that is often the roof window. Our skylights and roof windows page shows where to start.