The right answer usually depends on whether your constraint is light or height — and on what the front elevation can get away with.
Daylight versus usable space
A rooflight facing the sky gathers substantially more light than an equivalent dormer window. If the loft already has adequate headroom over enough of its area, rooflights alone can make an excellent room.
Where the ridge height is marginal, a dormer is the only way to create standing height and a usable floor area, particularly around a stair head or an en-suite.
Planning and permitted development
Rooflights that sit close to the roof plane are generally the least contentious addition, and are often acceptable where dormers are not. Rear dormers frequently fall within permitted development on unlisted houses, subject to volume and position limits.
Front-facing dormers, conservation areas, Article 4 directions and listed buildings all change the picture, so the design should be tested against the constraints before drawings are finalised.
Cost and construction
A rooflight is a comparatively quick operation: form the opening, trim the rafters, flash it in. A dormer is a small building — structure, roof, walls, insulation, cladding or tiling and its own weathering details.
The most common outcome we build is a hybrid: a rear dormer for headroom where the stair and bed space need it, with rooflights on the front slope for light without altering the street elevation.
Common questions
Do rooflights need planning permission?
On most unlisted houses outside designated areas they fall within permitted development when they sit close to the roof plane, but designations change that.
Which gives more usable space?
A dormer, without question. It creates standing headroom and floor area; a rooflight only adds light.
Are dormers much more expensive?
Yes, because a dormer is a small structure with its own roof, walls and weathering. Rooflights are a fraction of the cost.
Can I have both?
That is the most common arrangement — a dormer where headroom is needed, rooflights elsewhere for daylight.