Question answered
Roof window or dormer: which is right?
The short answer
Choose a roof window when the loft already has enough headroom and you want light and ventilation. Choose a dormer when you need more standing height and floor area. Roof windows fall under Class C of the permitted development rules and dormers under Class B. A dormer needs a planning application on article 2(3) land, which takes in conservation areas and National Parks, while a roof window often does not.
A roof window lets light in without changing the shape of the roof. A dormer adds floor space and headroom by building out from it. The planning rules, the cost and the result differ widely.
What each one actually does to the room
A roof window is set flush into the slope. It brings in daylight and lets you open the room to fresh air, but it does not move a single wall. If the loft already has a good clear height under the ridge, a window may be all you need.
A dormer is a built-out box on the slope, with its own upright cheeks and usually a small flat or pitched roof. It turns a sloping ceiling into usable headroom and floor space, which is why it features in loft conversions where a stair, a bed or a shower has to fit.
Think about where you will stand. Headroom only counts where you can walk, and a slope that comes down steeply to the eaves wastes the floor beside it. A dormer claws some of that back, whereas a roof window leaves the geometry as it is. The choice therefore starts with a tape measure inside the loft, not with a catalogue.
Class C for roof windows: what the rules say
Roof windows come under Class C of Part 1 of Schedule 2 to the General Permitted Development Order 2015, which covers other alterations to a house roof. Under C.1(b) the alteration may stand no more than 0.15 metres out from the original slope. Under C.1(c) its top may not pass the original roof's highest point.
There is one condition that catches people out. Condition C.2 deals with side slopes. A window there needs obscure glass, and it has to stay fixed unless the sash that opens is over 1.7 metres up from the floor. That height eases only the non-opening rule. Obscure glass is needed every time.
Class C has no exclusion for conservation areas or National Parks, so it can still apply there, though an Article 4 direction, a past planning condition can remove it, and so can listed status. Ask the Adur & Worthing planners, or the Park authority for a house inside the National Park.
Class B for dormers: where permitted development stops
Dormers and other roof additions come under Class B. The limits are tighter. Under B.1(b) no part of the addition may rise above the existing ridge height. Under B.1(c) it may not project beyond a principal-elevation slope that faces a road. Under B.1(d) extra volume is capped: 40 cubic metres for terraces, 50 for other houses.
The decisive line is B.1(f): the right does not apply if the dwellinghouse is on article 2(3) land. That land includes conservation areas and National Parks. So in a conservation area or the South Downs National Park a dormer needs a planning application, however small it is. Class B.2 adds that materials must be of a similar appearance and that the edge of the enlargement should, so far as practicable, sit not less than 0.2 metres from the eaves.
Where the house is a flat, neither class applies, as our page on adding a roof window to a flat explains.
How the cost compares
A standard roof window is the smaller job. MyBuilder's Velux windows cost guide (updated 21 August 2026) gives a typical UK range of £350 to £600 for a standard installation, labour and fitting included, with scaffolding at approximately £500. Those figures cover a straightforward swap or a simple new opening, not a conversion.
A dormer is a small building project. It involves structural timber, new roof covering, flashings, insulation, a window and often internal finishes, and its price turns on size, access, the roof's construction and what the survey finds. For that reason there is no single sensible typical figure to quote here, and we price it on survey with a written quote afterwards.
A dormer is usually the larger spend by a wide margin, so it must earn that spend in usable space. Our guide to rooflight prices lays out how sizes and glazing choices move the figure for the smaller option.
Light, ventilation and the weather
Both options also change how the room behaves. A roof window gives daylight from above, which fills a room more deeply than a wall window of the same size, and an opening one clears stale air quickly. A dormer gives a vertical window with a view out and a ledge you can sit by.
Both are weak spots in the roof if badly fitted, and on this coast they take a pounding. A dormer has more junctions than a window: cheeks, a roof, valleys or soakers where it meets the main slope. Every one is a possible way in for water, so the lead and flashings need careful dressing. A window has fewer joints, but its flashing must match the covering around it.
If noise matters, read whether roof windows are noisy in the rain. Our skylights and roof windows page covers the wider work. A roof window also asks less of the roof structure than a dormer does, which is worth remembering before the survey, so describe any sagging or damp to us first.
Windows against dormers: two more questions
Do I need building control approval for either option?
Ask your building control body before work starts, because a loft conversion that includes either option raises separate points about structure, insulation and escape. Planning permission and building control are separate systems, and passing one does not satisfy the other.
Can I fit a roof window in a conservation area without planning permission?
Often yes, because Class C has no conservation area exclusion, but an Article 4 direction or a condition on your house can take the right away. In the Old Shoreham and Kingston Buci areas, for example, directions remove it for roof slopes fronting a highway. Check with the planning team.
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