The window is the easy part.
"Velux" is a brand, not a product. The Danish manufacturer happens to be the market leader, and people say Velux the way they say Hoover. FAKRO, Keylite and RoofLITE all make comparable windows, often for less at a similar spec — and the fitting cost is the same whichever you choose. Being straight about that up front is the difference between a tradesman and a salesman.
Roof window or rooflight
Worth knowing before you buy. A roof window opens — for ventilation, and, in a loft conversion, for escape. A rooflight is fixed; it just lets light in. They're different products for different jobs, and the price reflects it.
Fitting matters more than the window
The window is the easy part. What keeps the water out is the flashing kit matched to your tile type, the timber trimming round the opening, and the way it's weathered into the surrounding roof. A badly flashed roof window leaks at the head — and the water tracks down inside the rafters, so it shows up somewhere else entirely and gets blamed on the roof.
We've pulled out plenty of "leaking Velux" jobs where the window was fine and the original flashing was the problem. Nine times out of ten, that's where a roof window fails — not the glass, not the frame, the flashing.
The rules, in plain English
Usually you don't need planning permission. A roof window is permitted development as long as it doesn't project more than 150mm from the roof plane and doesn't sit above the ridge. Conservation areas and listed buildings are the exception, and listed buildings need separate consent.
Building Regulations apply regardless. Approved Document L covers energy — a maximum U-value of 1.4 W/m²K for a replacement and 1.2 for a new opening. Approved Document F covers ventilation.
Cutting a new opening means trimming rafters, which needs structural calculations. And if you've got a trussed roof — most houses built from the 1960s on — the trusses can't be cut without an engineer's input. Never let anyone cut a truss without one.
Where we fit them round here
Loft rooms and conversions across Walton and Brockwell and up into Ashgate. Dark landings and back bedrooms in the Victorian terraces through Whittington Moor and Newbold. Bathroom and kitchen extensions everywhere. And plenty of the older ones fitted twenty-odd years ago are now leaking at the flashings and due a replacement rather than a repair.
The honest bit
If you've already got a roof window, replacing it is cheaper than a first-time fit, because the structural opening already exists. A like-for-like swap usually runs £700 to £1,200 including new flashings. We always renew the flashings at the same time, because that's where the leak normally is — fitting a new window to old flashings is a false economy.
We fit Velux, FAKRO and Keylite — the three names worth fitting — and RoofLITE where the budget matters. They're all good windows. The fitting is the same; the price isn't, and we'll show you the options at the spec you want.
Types: centre-pivot is the common one — it rotates 180° so you can clean the outside from inside. Top-hung opens wide at the bottom and is what you'd use where the window is an escape route. There are electric and solar versions with rain sensors that close themselves, and conservation-style windows for period and listed property.
Pitch matters. Standard roof windows suit 15° to 90°. Low-pitch versions go down to 10°, and flat-roof units cover 0° to 15°. We'll confirm your pitch on the quote and tell you if it limits the options — there's no point fitting a window the roof's too shallow for.
The flashing kit is matched to the tile — plain tile, interlocking, slate, or a profiled sheet. Get it wrong and it leaks; get it right and it outlasts the window. We always fit the manufacturer's matched kit, never a generic one.
| Standard roof window, supplied and fitted | £900–£1,800 |
| Like-for-like replacement (opening exists) | £700–£1,200 |
| The window unit itself | £350–£900 |
| Flashing kit | £100–£250 |
| Labour, 1–1.5 days per unit | £350–£600 |
| Electric or solar opening | add £300–£800 |
| Triple glazing | add £250–£500 |
| Conservation-style | add 15–25% |
| Flat-roof rooflight | £1,800–£3,500 |
| Scaffolding, where needed | around £500 |
A replacement is cheaper than a first-time fit because the opening already exists — and we always renew the flashings at the same time, because that's where the leak normally is. Fitting a new window to old flashings is a false economy.
