A rooflight (Velux-style) conversion works within the existing roof shape, adding windows but no extra volume. A dormer adds a vertical-walled box that projects out from the roof slope, creating genuine standing head height and floor area that wasn't there before.
The head-height test comes first
Measure from the top of the ceiling joists to the underside of the ridge board. If that figure is comfortably above standing height with a little to spare for insulation and floor build-up, a rooflight conversion can deliver a usable room without any structural roof alteration.
If the ridge height is close to or below that threshold, a rooflight conversion will leave you crouching through most of the room, and a dormer becomes necessary just to create somewhere you can stand.
What a dormer adds beyond head height
The vertical walls of a dormer create proper corners for a bed, wardrobe or bathroom fittings to sit against — furniture doesn't work well hard against a sloped ceiling, so a dormer significantly increases genuinely usable floor area, not just headroom.
That extra volume is usually what makes fitting a full bathroom into a loft conversion realistic, since a shower enclosure or bath needs standing height and a reasonably square footprint that a sloped roof alone rarely provides.
Cost and planning differences
A rooflight conversion is considerably cheaper and quicker because it doesn't alter the roof structure — new joists, insulation, a stair and rooflights, with no new roof covering or dormer cheeks to build.
Dormers involve altering the roof structure and cladding a new box in matching or contrasting materials, so they take longer and cost more, and depending on size and position they can trigger a planning application rather than sitting under permitted development.
Visual impact
Rooflights sit flush with the roof slope and are barely visible from the street, which matters in a conservation area or on a prominent roof.
A dormer changes the roof's silhouette, which some households want as a deliberate visual statement and others try to avoid, particularly on the front elevation facing the road.
Common questions
How much head height do I need for a rooflight-only conversion?+
It depends on the room's intended use, but as a working principle you need standing height across a reasonable proportion of the floor once insulation and boarding are accounted for — we measure this specifically on survey rather than quoting a blanket figure.
Can a dormer be added to the rear only, keeping the front roof unchanged?+
Yes, this is the most common approach on a semi or terrace — a rear dormer adds volume where it's least visible from the street while the front slope stays untouched.
Do all dormers need planning permission?+
Not necessarily — many rear dormers fall under permitted development within set size and position limits, but front-facing dormers, conservation areas and listed buildings all change that, so it needs checking property by property.
Get your loft head height assessed
We'll measure the ridge height on survey and tell you honestly whether a rooflight conversion works or whether you need a dormer.