Short answer: The window is just one of six things that set the cost to fit a roof window. They are whether the job is a new opening or a swap, the size, the way it opens, the flashing for your roof covering, the route to the roof, and the internal lining. Replacing a window in an opening that already exists is the simplest job. A new opening in a rafter roof is the most involved, and the only honest number comes after a survey, as a fixed written quote.
New opening or swap: the biggest dividing line
Our reply to anyone asking what it costs to fit a roof window begins with a question of our own: is there already an opening in the roof? A swap goes into an existing opening, so the rafters, the trimmers and the lining are already there. The old unit comes out, the flashing is checked against the new frame, and the new one goes in. A good deal of the structural work is already done.
A new opening is a different job. The roof covering is stripped over the area, one or more rafters are cut, and the load they carried is taken by new trimmers fixed above and below the opening. Then the frame, the flashing, the underlay collar and the covering all go in as fresh work. Inside, the reveals and the plasterboard around the opening have to be built, boarded and finished. The roof window price you are given reflects all of that labour as well as the unit.
The type of roof matters too. A cut-rafter roof, typical of Victorian and Edwardian terraces, takes trimmers readily. Trussed rafter roofs are the type found on estates from the late 1960s onwards, and their timbers cannot simply be cut. An engineer or the truss maker has to design the opening. That adds a step before the work can start.
Size, make and how the window opens
How the window opens changes the price as well. A fixed window is the plainest option. A centre-pivot window, which turns about its midpoint, is the usual opening type. Top-hung and side-hung models cost more because the mechanism is more involved, and escape models, where the sash must clear a specific opening size, sit towards the top of the range. A motor, a rain sensor and a controller come with electric and solar windows, and the electric version also needs power run to the opening. A blind has side channels attached to the frame, and it is priced separately from the window.
The make matters less than people expect. Comparing like with like is the useful exercise: the same size, the same opening type, the same glazing and the same flashing, as set out in a written quote. If a price looks very different from others, check that each quote covers the same items. For the make-specific picture, see the VELUX installation cost answer.
| Factor | Simpler | More involved |
|---|---|---|
| Opening | Swap in the same hole | New opening with trimmers |
| Roof structure | Cut rafters | Trussed rafters |
| Window action | Fixed or centre-pivot | Top-hung, escape or electric |
| Access | Reached from inside | Scaffold needed |
Flashing and the roof covering
The flashing kit has to be made for the covering the window sits in, so slate, plain tile and profiled tile each call for a different one. A recessed fitting, where the glass sits low in the roof, needs a different kit from a standard one and more work on the rafters. The kit is a small part of the total, but the labour of lifting, cutting and relaying the covering around it is not.
The state of the roof around the opening changes the work as well. Old slates that split on the way out are replaced with matching ones. A brittle or missing underlay near the opening is made good so the new window ties into something sound. We look at all of this at survey, so the quote does not grow once the work begins. The detail is covered in the guide to flashing a roof window in slate and tile.
Getting to the roof, and finishing inside
Access is a real cost. Many roof windows can be fitted largely from inside the house, with the covering handled from a short ladder or a small platform. A window high on a steep slope, or on a roof that cannot be reached safely any other way, needs scaffold, and scaffold has to be erected, hired and taken down. On a terraced street with no frontage or a shared passage, it can also need arrangements with neighbours or the council.
- New opening or swap
- Window size and opening type
- Flashing kit for the covering
- Scaffold or inside access
- Lining, plastering and decoration
- Building Control notification
Paperwork, and how a quote is built
Fitting a new or replacement roof window is controlled work under the Building Regulations, so the paperwork goes to the council, or else to a registered building control approver, and we handle it where the work is notifiable, so it is not a separate errand for you. Because the glazing has to meet the thermal limit for existing dwellings, the specification is chosen with that in mind from the start.
In a fair quote, the window, flashing kit, structural work, access, lining and notification each get their own line, which lets you see where your money goes. Ours follows a survey, and the figure is fixed and in writing before any work is booked. When the job is finished you also receive a written guarantee on our workmanship, lasting ten years. Booking the survey takes a phone call, the form on this page, or a WhatsApp message with photos of the roof and the room. Get a free quote
For the wider picture of choosing a window, see roof window installation, and for how prices are put together across our other services, the cost guide.
Cost questions before a survey
Why can't you give a price over the phone?
Because the roof decides it. The covering, the pitch, the rafters, the access and the state of the room inside all change the work, and none of them show on a phone call. A survey takes in all of them and lets us give one fixed figure in writing.
Is swapping an old roof window cheaper than adding a new one?
Usually, yes, because the opening, the trimmers and the lining are already there. The old unit comes out and the new one goes in with a fresh flashing kit. If the new window is a different size from the old one, some of the new-opening work comes back in.
Does a more expensive make last longer?
Not by itself. How long a roof window lasts depends on the quality of the unit and, just as much, on the flashing and fitting around it. We compare like with like in the quote, so the specification is clear whichever make you choose.