The short answer
Choose a roof window if the loft already has enough head room and you mainly need light and ventilation; it is quicker, less disruptive and usually permitted development. Choose a dormer if you need more floor area with standing height, and accept more building work, a planning check and a higher cost. Dormers are not permitted development in conservation areas or the Cotswolds National Landscape.
What each one does to the room
A roof window lies in the plane of the roof slope. It lets in light and air but leaves the shape of the loft as it was. If the space already has useful head room under the ridge, a roof window turns the dark corner into a room with a view of the sky.
A dormer sticks out of the slope as a box with upright cheek walls, its own roof and normally a vertical window. By pushing the slope outwards, it creates extra floor area where you can stand, and room for a bed, a desk or a shower to be placed where the rafters used to be in the way. The cost is that a dormer is a small building, with its own structure, roof covering and waterproofing.
Think of it this way. A roof window changes how the room is lit. A dormer changes how much of the room you can use. If your loft is already generous, a dormer is rarely worth the trouble. If it is a steep, tight triangle, a roof window will give you light in a space that still feels cramped.
Head room and layout
Measure the room before choosing. Stand along the ridge line, where the ceiling is highest, and then move towards the eaves. The width of the strip where you can stand upright tells you how much of the floor is truly usable. Anything below that is storage or a low bed, not a room.
A roof window can be set so that the glass starts at a comfortable height, and it can be placed to light the standing zone. Several windows in a row bring light across a long loft. They do not move the walls, though, so the loft layout remains what the roof allows.
A dormer helps when the usable strip is too narrow. Because its walls are vertical, the floor area directly below it becomes fully usable up to the dormer's cheek walls. Stairs often need the extra headroom too, which is another reason a dormer is placed over the stairwell in many conversions. Our answer on the alternatives to a skylight lists other options, such as sun tunnels and gable windows, if neither of these suits.
Planning: the line between the two
Planning is where the two options part company in this area. Roof windows fall under Class C of the General Permitted Development Order. Within its limits, a unit may not stick out more than 0.15 metres beyond the roof plane and may not end up higher than the original ridge. Class C has no restriction tied to article 2(3) land, so roof windows can be permitted development in a conservation area or the Cotswolds National Landscape, unless an Article 4 direction or a condition on an earlier permission takes the right away. A listed house needs separate consent.
Dormers are dealt with by Class B, and that class does not apply to a house on article 2(3) land. That means conservation areas and the Cotswolds National Landscape, which covers just over 50% of Stroud District's land area according to the district's own profile. Stroud District also has 41 conservation areas. A dormer in any of these places needs a planning application, and a dormer elsewhere still has to meet the Class B limits.
Listed buildings need listed building consent for alterations that affect their character. For anything near a boundary of a designated area, check with Stroud District Council's planning team before you design anything.
Building Regulations and escape
Either option brings Building Regulations into play, mainly through two parts. A new or replacement roof window has to meet Approved Document L, with a limit of 1.4 W/m²K or Band B. A dormer is a new structure, so its roof, cheeks and floor have to meet the insulation limits for new elements, with the roof set at 0.15 W/m²K in Table 4.2, and the timbers have to carry the load. Our security answer also touches on what an opening window means for a loft room.
Escape matters in a loft that becomes a bedroom. Approved Document B covers houses with a single stair: each habitable room needs an emergency escape window or a way into a protected stairway. Such a window needs an openable area of 0.33 m² with a clear height and width of at least 450 mm each, and its lowest edge no more than 1100 mm up from the floor. A roof window can only serve as the escape route where the floor is within 4.5 m of the ground. A dormer window, being upright, can often meet the sill height more comfortably.
On notifiable work our office takes care of the Building Regulations notification and tells you which consents apply before the job starts.
Cost, disruption and heat
A roof window is the lighter job. MyBuilder's 2026 guide puts fitting at £350 to £600 for standard jobs, with a basic centre-pivot unit at £305 and scaffold about £500. It may involve trimming a rafter or two, a day or so on the roof and some internal finishing. Because the work is small, the covering is open for a short time.
A dormer is a larger piece of building. The roof is opened wide, new structure goes in, and the dormer has to be waterproofed, clad and insulated. We do not quote a typical national figure for it here, so it is priced after a survey, with the quote covering the scaffold, structure, covering and finishing. If the dormer has a flat roof, our skylight overheating answer is worth reading for the heat side, and dormer roofs themselves carry the same questions about membrane and lead as any flat roof.
Heat is a real consideration. Large areas of glass in a south-facing slope warm the room quickly in summer, whichever you choose. A dormer's upright window is shaded by its own roof; roof windows can take blinds or solar-control glass.
Like for like applies to the covering: stone stays stone, slate stays slate. Following our survey and photographs, the written quote covers the whole job, and the work has a written 10-year workmanship guarantee. The wider range is on our roof windows and skylights page.