Velux Loft Conversion Cost Calculator
A Velux conversion leaves the roof line alone, which is why it is the cheapest route and usually stays inside permitted development. It also gives you the least head height, so the ridge height is the first thing to check.
Open the calculator →Price your rooflight conversion in 30 seconds
4 steps. Room size, staircase route, en-suite and region.
How this calculator works
A Velux conversion is the roof you already have, insulated, floored and lit from above. Nothing is added to the structure, which is why it is the cheapest route and why the existing ridge height decides whether it works at all.
The calculator prices the room, the staircase route and the en-suite separately because those are the three things that actually move a Velux quote. Rooflight count matters far less than people expect — each extra one is £900–£1,800 fitted.
On regulations: permitted development covers planning, not building control. The conversion is notifiable either way, and Part B fire safety is where most of the unexpected cost sits.
Velux loft conversion cost by room size (2026)
| Option | Typical 2026 cost | Notes |
|---|---|---|
| Single room, up to 15 m² | £22,000–£28,000 | Office or single bedroom |
| Standard double, 15–22 m² | £27,000–£34,000 | The usual Velux conversion |
| Large room, 22–30 m² | £32,000–£38,000 | Wide detached roof or bungalow |
| Cost per m² (rooflight spec) | £1,200–£1,800 | Lowest of any loft type |
| Each additional rooflight | £900–£1,800 | Supplied and fitted, flashing included |
| 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 Velux loft conversion cost guide. Building control applies even when planning permission does not.
Velux loft conversion calculator FAQs
On the same roof, yes — normally £8,000 to £25,000 less, because nothing is added to the roof structure. The trade-off is head height and usable floor area, and on a shallow roof a Velux conversion can leave you with a room you cannot stand up in at the edges.
Usually not. Rooflights that sit no more than 150mm above the roof plane and do not go above the ridge normally fall inside permitted development. Flats, maisonettes, listed buildings and conservation areas are the exceptions — see our planning guide.
You want at least 2.2m at the ridge before a Velux conversion is comfortable, because the floor build-up and insulation will take 150–250mm of it. Building Regulations only require 1.9m at the centre of the stair width, but a room at the legal minimum feels cramped.
Six to eight weeks on site for a standard Velux conversion, against ten to twelve for a dormer. Most of the work is inside, so weather delays are rarer.
Related cost guides
- Loft conversion costs — every type priced
- Planning permission — the PD volume limits
- Loft conversion quotes — 3 free quotes