Dormer Loft Conversion Cost Calculator
A dormer buys you full head height across the new room, which is why it is the default choice on a terrace or semi. It is also the point at which the roof structure changes, so the price step from a Velux conversion is real.
Open the calculator →Price your dormer in 30 seconds
4 steps. Dormer size, staircase route, en-suite and region.
How this calculator works
A dormer replaces part of the sloping roof with a vertical wall and a flat roof above it. That is what gives you full head height across the room, and it is also why the price step from a Velux conversion is a structural one rather than a finish one.
Size is the main lever, and it is not linear — a full-width dormer is not twice a single one, because the scaffolding, the steels and the roof covering all scale differently from the fit-out.
On planning: permitted development is a volume allowance for the whole house, not per project. If a previous owner already dormered or extended, some of it is gone — a Lawful Development Certificate is the way to be sure.
Dormer loft conversion cost by type (2026)
| Option | Typical 2026 cost | Notes |
|---|---|---|
| Single rear dormer | £30,000–£45,000 | One box over part of the roof |
| Double / full-width dormer | £45,000–£65,000 | The whole rear roof |
| Dormer plus hip-to-gable | £50,000–£70,000 | Semi or end-terrace |
| Cost per m² (dormer spec) | £1,500–£2,500 | Above Velux, below mansard |
| Flat-roof dormer, single | £18,000–£28,000 | Where the dormer is the whole job |
| Winder or quarter-turn stair | +£2,000–£4,000 | Where the landing is tight |
| En-suite, stack reachable | +£4,000–£7,000 | Rises sharply if it is not |
Figures match our dormer loft conversion cost guide and flat-roof dormer costs.
What changes a dormer quote the most
The single-versus-full-width choice sets the floor, but three things move a real quote inside the ranges in the table above. The first is the steelwork. A dormer needs beams to carry the new flat roof and, once the slope is opened up, often the ridge as well; a wider opening or a longer clear span means heavier sections and a harder lift, which is why a double dormer costs more than twice a single one rather than exactly double.
The second is the party wall. On a terrace or semi the new structure almost always sits within three metres of the boundary, so the Party Wall etc. Act 1996 applies and you will need a signed agreement — frequently a surveyor acting for each side. Budget for the notice period as much as the fee: served late, the two-month notice can push your start date back further than the build itself.
The third is access and weather. The roof stays open under a temporary cover for part of the job, so the scaffold is up for the full ten to twelve weeks and a winter start carries a real risk of delay. A rear dormer reachable only through the garden — or a scaffold that has to bridge a neighbour’s roof — adds design and sometimes a licence, and none of that shows up in a headline per-square-metre figure. If your semi or end-terrace also needs the gable built up first, price it with the hip-to-gable calculator.
Dormer loft conversion calculator FAQs
Usually, within the volume allowance — 40m³ on a terrace, 50m³ on a semi or detached house, measured across the whole roof and reduced by anything already built. Front-facing dormers on the principal elevation are not. Check our dormer planning guide.
Because the whole rear roof comes off rather than part of it. That means more steel, a bigger flat roof, more scaffolding time and usually two rooms fitted out instead of one.
Flat is cheaper and gives more internal height, which is why most rear dormers are flat. A pitched dormer looks better from the street and is often what a conservation area officer will want.
Ten to twelve weeks on site is typical. The roof is open under a temporary cover for part of it, so a winter build carries more weather risk.
Related cost guides
- Loft conversion costs — every type priced
- Planning permission — the PD volume limits
- Loft conversion quotes — 3 free quotes