New opening or replacement
The biggest split is between putting a window where there was none and swapping one that is already there.
A swap into an existing opening usually means removing the old unit and its flashings, checking the frame timbers, fitting the new window and weathering it again. The hole is already the right shape, so there is less cutting and less making good.
A new opening is more work. The covering has to be stripped back, a rafter may be cut and a trimmer fitted to carry the load, the opening has to be framed, and the lining inside built from scratch. The ceiling or sloping rafters need opening up and finishing as well. All of that is labour and materials the swap does not need. If you are replacing one VELUX with another and the old one is a standard size, the job tends to sit at the simpler end.
Both kinds of job are covered on our VELUX windows page.
Size and the number of windows
A larger window costs more than a smaller one. The unit itself is bigger, the opening is wider, the flashing kit is larger and there is more to line inside. Sizes carry a code such as MK04, and our size codes guide shows how to read it.
Number matters too, though not in a straight line. Fitting two together saves time, and the total still grows with each one added. Two windows side by side on the same slope share set-up time, scaffold or access, and a single visit. A pair or a row is cheaper per window than two separate jobs, though the total is higher than for one. The rafters left between neighbouring windows also set how wide each can be.
The roof covering and the pitch
The roof covering around the opening has to be cut back, then rebuilt around new flashings. How long that takes depends on what the roof is made of.
- Plain tiles are small and need careful cutting and relaying around the frame.
- Interlocking tiles are larger, so there is less cutting, but the tile profile has to match the flashing.
- Slate is brittle and can crack when disturbed. Old slate takes time to lift and refix, and replacement slates may be needed.
- Other coverings such as metal or single-ply membrane have their own flashing detail.
The pitch matters as well. Very shallow slopes need a different approach, because water runs off slowly. See roof windows for low-pitch roofs for the detail. A steep, high or awkward roof takes longer to work on.
The flashing kit that goes with the window is chosen for the covering, so the covering also affects which parts are specified for the job.
Access, height and the working set-up
Some roofs are easy to get at: a rear slope on a low house with room for a ladder. Others need more. A third storey, a roof over a conservatory, a terrace with no space at the foot, or a slope only reached from inside the building each add set-up time.
Where scaffold or a tower is needed to work safely, that is a cost the job carries on top of the window. It also matters for the materials: carrying a large window up several floors takes more people and more time than lifting it onto a low roof.
Access is a good example of why a price over the phone for a VELUX window is a guess. Two houses on the same street can be quite different jobs: one has a clear side passage and a low rear roof, the other a tight yard and a third-floor slope. The ladder, the scaffold and the path through the house are all things we see on a survey.
Glazing, controls and extras
The window you choose changes the figure. Glazing options such as thermal, solar control, acoustic or safety glass are specified per range, and they differ in what they offer and what they cost. Electric opening, rain sensing and blinds are extras on top of the basic unit, and each needs fixing and sometimes wiring. A window with a power supply needs a cable run to it, and electrical work has its own rules.
Existing homes have a Part L target for roof windows, so the glazing specification is more than a comfort choice. Our guide to planning and Building Regulations sets out where that applies.
Finishing inside and getting a fixed figure
The inside of the opening is part of the job. A loft room with exposed rafters needs lining, insulation and plastering or boarding round the reveal, so the new window finishes cleanly. If a ceiling has to be opened up, the plaster, filling and decoration afterwards are further work to allow for, and the level of finish you want changes how long it takes.
Compliance can add a step. Where a job is notifiable, the Building Control paperwork is ours to deal with. Planning rarely needs an application for a roof window.
The way to get a clear figure is a survey. It is free. We check the roof, the room and the route in, take the measurements, then write up one fixed quote naming the window, the flashings and the finish. Nothing is left as an estimate to be settled later. For the wider picture on how we think about pricing, see our costs page, then ask for a survey or message us on WhatsApp.