How much does a roof window cost to fit?
Between rafters is the cheapest useful job in this trade. Cutting a truss is a different order entirely.
Part of our Roof Window Installation guide
Roof windows have the widest cost spread of anything we fit, because the same window in two different roofs can be two very different jobs.
What moves it
Whether it fits between rafters. If it does, there is no structural work at all and this is the cheapest useful job in this trade. If it does not, a rafter is cut and trimmed.
Cut roof or trussed roof. Trimming a cut roof is routine carpentry. Cutting a trussed rafter needs a structural engineer’s replacement design, and that changes both cost and timescale materially.
The covering. Slate and plain tile are straightforward. Profiled tiles need a deep profile kit. Where tiles are obsolete or fragile, sourcing replacements for the ones cut adds cost.
Access. A single storey rear slope may be reachable from a tower. A two storey front slope generally means scaffold.
Internal finishing. Forming and plastering the reveal is a real part of the job, and a splayed reveal is more work than a square one and throws noticeably more light.
The false economy
Choosing a larger window that does not fit between rafters, to gain a modest amount of glass, and paying for structural work to get it. On a trussed roof that decision can double the cost of the job.
How we price is on the costs page.
What the range depends on
We give an indicative range by email from a photograph and a fixed written price after survey, because the spread on this job is genuinely wide and a single number would mislead.
Five things move it.
Whether the opening already exists. The single biggest factor. A like for like replacement into a sound opening is the cheapest job in this trade. Forming a new opening means cutting and trimming structure, and altering an existing one sits between the two.
Access. Frequently the biggest surprise on a quotation and nothing to do with the window. A single storey rear slope reachable from a tower costs nothing extra. A second storey, or a front elevation over a pavement, generally means scaffold, and where that scaffold stands on the highway a licence is needed too.
The unit. Stock size, obsolete size or made to measure, with the glazing specification on top. Solar control, acoustic laminate, obscure and triple glazing are each a step up.
The covering. Slate and plain tile are straightforward. Profiled tiles need a deep flashing kit rather than a flat one. Handmade or fragile clay tiles that break when lifted have to be sourced or robbed from a hidden slope.
What is found. Soft timber around the opening has to be cut out and replaced. On a unit that has been failing quietly for years it is a real possibility rather than a remote one, and an honest quotation names a contingency for it.
The reliable way to spend less
Do several at once. Access is a fixed cost shared across everything done in the same visit, so two windows fitted together are meaningfully cheaper than two fitted a year apart.
On replacements this matters more than it sounds, because units fitted at the same time fail at about the same time. If two of three are misted, the third is not far behind, and doing it while the access is up costs a fraction of doing it separately later.
What should be on the quotation
Eight lines: the unit with manufacturer, model, size code and glazing specification; the flashing kit and which profile; the insulation collar; access and who provides it; making good, to what standard, and whether decoration is included; disposal; Building Control where notifiable; and a stated contingency for decayed timber.
A quotation that says roof window supplied and fitted with one number against it is not comparable with anything. Where two quotations differ by a factor of two, the difference is almost always in what is not written down.
Where cheap quotations save money
Three places, none visible when the job is finished. A universal flashing kit rather than the correct profile. No insulation collar, which shows as reveal condensation months later. And making good that stops at the plasterboard rather than including plastering and decoration.
What we do
An indicative range by email from a photograph, free and without an appointment, so you know whether the project is viable before anybody visits. Then a fixed written itemised price after survey, which does not move unless the job changes or something we told you might be found is found.
We would rather lose a job on an honest number than win it on one that grows.
Ask us
Send a photograph of the roof, one of the frontage so we can see the access, and the data plate if you are replacing. That is enough for a realistic range by email, free and without an appointment.
Where the quotations differ most
Access, almost always. The unit price is broadly comparable between suppliers; what varies is whether scaffold is included, whether a highway licence has been allowed for, and whether making good stops at the plasterboard or includes plastering and decoration. Those three account for most of the spread.
One last point
Doing several at once is the reliable saving. Access is a fixed cost shared across everything done on the same visit, and units fitted at the same time fail at about the same time anyway.
Why two quotations for the same window differ
Almost always in the lines around the unit rather than the unit itself.
Access. Scaffold or no scaffold is frequently the single largest difference on a small job, and it is the first place to look.
The insulation collar. Cheap, invisible once fitted, and routinely omitted. Ask for it by name.
Making good. Whether plastering the reveal is included, and whether decoration is.
Disposal. Removal of the old unit and glass is not free.
Timber allowance. A quotation that says nothing about the condition of the trimmers is either optimistic or leaving you exposed on the day.
How to compare properly
Ask every quoter for the same breakdown: unit specification including whole unit U value, flashing kit and profile, collar, access, making good, disposal, VAT.
Once those are separated, the comparison is real. A single lump figure with none of it broken out cannot be compared with anything, and that is usually where the missing items are hiding.
More on Roof Window Installation
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