The short answer
It varies widely, so a single figure would mislead. A small fixed dome costs far less than a large opening glass rooflight, and a walk-on unit or a lantern costs more again. Besides the unit, the price includes a kerb to raise it, the work to tie it into the flat roof covering, any structural trimming and finishing inside. Ask for a quote that itemises each part, and compare them like for like.
Why no honest single price exists
You will find prices quoted for flat roof skylights across a wide spread, and many of them come from lead-generation sites with no source. We do not have a reliable published range for flat rooflights specifically, so we would rather not make one up. What we can do is explain what moves the price, so you can judge a quote. For pitched roof windows, third-party cost guides give a rough anchor: a centre-pivot window supplied and fitted is commonly reported at about £680 to £1,300, and one manufacturer's own guide gives about £1,623 to replace and £1,906 for a new one including VAT. Flat rooflights are a different product, and those figures are only a loose comparison. Your fixed quote follows the survey.
The unit itself
The first driver is the type. A fixed rooflight lets in light and does not open, which makes it the simplest and cheapest. An opening one adds hinges, a handle or a motor and ventilation. An electric opening unit with a rain sensor adds more. The guide to flat roof rooflights shows the types: domes, pyramid or flat glass units, walk-on glazing and lanterns.
Size is the next. Price rises with area, and not in a straight line, because larger units need stronger frames and thicker glass. Glazing counts: a single skin or a double-skinned polycarbonate dome is cheaper than a sealed double-glazed glass unit, and triple glazing, laminated or solar-control glass costs more. Our answer comparing a roof lantern and a flat rooflight sets out the price difference in outline.
The kerb and the roof work
Flat rooflights sit on a kerb, a raised upstand around the opening that lifts the glass above the roof surface so water drains away. The kerb may be supplied with the unit or built from timber and then covered. Building and insulating the kerb takes time, and it is a part of the cost people forget. The waterproof covering of the flat roof must be dressed up the kerb and sealed, which means the roofer, not just a glazier. If the roof is EPDM, GRP or felt, each needs its own detail, and a new rooflight is a good moment to check the covering is in good order.
The roof structure affects the price too. If the opening cuts the joists, they must be trimmed and supported, as with a pitched roof. The work may need a calculation. A larger opening, or one that moves a joist, adds structural cost. Our page on the flat roofing service describes the coverings and what each needs.
Inside the room
The finish below the opening matters. A light well, the lined shaft between the ceiling and the roof opening, is built and plastered, and in a thick roof it can be deep. That adds materials and decorating. Wiring for a motor or a rain sensor, if you choose them, is extra and needs an electrician. Scaffold is rarely needed on a single-storey flat roof, but access, parking and crane lifting for a large glass unit may be.
Rules that affect the specification
New rooflights are notifiable under the Building Regulations, and the thermal standard for rooflights in existing dwellings is 2.2 W/m²K. A cheap single-skin dome will not meet it. The compliant, insulated units cost more, and the paperwork and inspection add a little. Our guide to roof windows and building regulations sets out what applies.
Fixed, opening and walk-on, in terms of cost
As a rule of thumb, the order from least to most expensive runs: a small fixed dome, a larger fixed glass unit, an opening unit, an electrically operated opening unit, a lantern and then a walk-on glass floor. The order can shift with size and glazing, but the pattern holds. Walk-on glazing is made of laminated, load-bearing glass and needs a stronger frame, and a lantern is a built structure rather than a unit. Each step up adds both material cost and installation time. It makes sense to decide what the rooflight is for, light or ventilation or access to a roof terrace, before choosing the type.
Comparing quotes
Ask each quote to state the make and size of the unit, the glazing and U-value, the kerb build and insulation, the covering work, structural trimming, internal lining and decoration, waste removal and the guarantee. A low figure that leaves out the kerb or the lining is not a bargain. The more a quote itemises, the easier it is to see what you are paying for.
We survey the roof for free, check the structure and covering, and photograph what we find. You get a written quote with the rooflight and each piece of work priced separately, so you can decide what to include. Other rooflight and window work is gathered under roof windows and skylights.
Every roof is different. A free survey with photos settles it for yours.
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