People ask what a flat roof skylight costs before they ask almost anything else. It is a fair question, and a single number would be a poor answer, because the figure is built from parts. Change one part and the total moves.
This guide takes those parts one at a time, in the order they affect a quote: the size of the opening, the glass, whether the unit opens, the upstand it sits on, the roof beneath it, access, and the finish inside. We do not print figures here. A rooflight is made for your roof, so the honest number comes from a survey, and we give it to you in writing.
Read it as a map of where the money goes. Once you can see the parts, you can decide which ones matter to you and which can be simpler.
Why one number would mislead you
A flat roof skylight is not a catalogue item that arrives in a box and drops into place. The glazed unit is made to a size. It sits on an upstand, which is a short insulated kerb raised off the roof deck. The kerb is covered by the roofing membrane, and the whole assembly has to drain, seal and insulate as one piece.
So the price has three layers. There is the unit itself. There is the work to prepare the roof around it. And there is the finish inside, where the opening meets the ceiling. A quote that names only the first layer leaves the other two as surprises.
That is why we survey first. We look at the roof from above, check the structure below, measure the opening and write down every layer. Your quote is written down and fixed, and that figure is the one we work to when we arrive.
Size sets the base
Bigger glass costs more to make, more to carry and more to lift into place. That part is obvious. What is less obvious is how size also changes the work around the unit.
A small fixed rooflight over a hallway may need one set of hands and a modest upstand. A wide rooflight over a kitchen island is heavier, may need a lifting plan, and can ask more of the roof structure under it. If the opening is wider than the existing joists allow, the carpentry grows: new trimmers, doubled joists, a stronger edge to the opening.
The useful habit is to start from the room, not the catalogue. Decide where in the room the sky should show, then size the unit to that. A rooflight that lights the table does more than a larger one that lights the corridor beside it.
| Choice | Raises the cost when | Keeps it down when |
|---|---|---|
| Size | The opening is wide and heavy | The opening fits existing joists |
| Opening type | It opens, vents or is motorised | It is a fixed pane |
| Glass | Extra coatings or toughening are added | A standard double glazed pane serves |
| Upstand | A tall kerb or custom build is needed | A low standard kerb is enough |
| Access | Scaffold or a lift is needed | The roof is easy to reach |
| Inside finish | A deep, splayed lining is wanted | A simple square reveal is fine |
Fixed, opening or walk-on
A fixed rooflight is a sealed pane in a frame. It is the simplest build, and it suits a room that already has windows to ventilate it. An opening rooflight adds hinges, a handle or a motor, and a frame that has to seal when closed. That extra engineering raises the cost, and it earns its place over a kitchen or a bathroom where steam gathers.
A walk-on rooflight is a different kind of unit again. It is built to carry a person, so the glass is thicker and laminated, and the frame is stronger. It suits a terrace or a roof you use, not a roof you only look at.
Ask yourself what the unit must do. If the honest answer is only to let light in, fixed is enough. If fresh air is the point, pay for the opening. If someone will stand on it, pay for the strength. Do not buy a job the rooflight will never do.
- Fixed: light only, the simplest build
- Opening: light and air, more parts and more engineering
- Walk-on: light and a surface you can use, the heaviest build
What the glass is asked to do
Glass is where the thermal performance lives, and it is where small upgrades add up. A double glazed sealed unit is the starting point. Coatings can reduce heat loss, and some reduce the sun's heat in summer. Laminated glass holds together if it breaks. Toughened glass is made to resist impact.
The Building Regulations limit heat loss through a flat rooflight in an existing house. Approved Document L, Table 4.2, gives the figure for rooflights in existing dwellings, and it is shown below. A unit that meets it is the floor, not the ceiling. If the room faces south and heats up, glass that manages the sun's warmth may be worth the extra.
If the room is a kitchen with a south-facing roof, think about glare and heat in July as well as brightness in January. A rooflight has no overhang to shade it, so the glass does the shading. Tell us how the room feels now and we will suggest a specification that suits it.
The regulation figure that applies to a rooflight
- 2.2 W/m²K
- ceiling for a rooflight in an existing home (Approved Document L)
- 1.4 W/m²K
- ceiling for a roof window, or WER band B
The upstand is half the job
On a pitched roof, a rooflight sits in the slope. On a flat roof it sits on an upstand: a kerb that lifts the glass clear of the water on the roof surface. The upstand is built from timber or a prefabricated kerb, insulated, and wrapped in the roofing membrane. It is also what gives the glass its slight fall, so rain runs off instead of pooling.
The height and the build of that kerb change the price. A low kerb on a clean, new roof is the simple case. A taller kerb, or one that has to be shaped to suit a roof that falls in two directions, takes more time and more material. If the existing roof needs lifting or re-laying around the opening, that work belongs in the quote too.
We cover the detail in our guide to upstands and falls. For cost purposes the point is simple: the visible glass is only part of what you are paying for. The kerb and its waterproofing are the part that keeps the room dry for years.
The roof underneath decides the surprises
Every quote rests on what is under the membrane. A sound deck with accessible joists is the easy case. A roof with soft boards, damp insulation or a failing covering is not, and it is better to find that at the survey than with the old rooflight half out.
Our survey includes a look at the structure, the build-up and the condition of the covering around the opening. If the surrounding roof is near the end of its life, we say so. Sometimes it makes sense to renew the covering around the new unit in the same visit, because the weatherproofing is joined together and a fresh kerb on a tired roof leaves a weak seam.
This is not an upsell. It is the difference between a rooflight that is watertight on day one and one that is watertight for the life of the roof. We write whatever is needed into the fixed quote, so nothing arrives as an extra on the day.
Access, height and the day itself
Getting glass onto a roof is part of the job. A bungalow extension with a low eave and a clear garden is straightforward. A flat roof at the back of a terrace, reached only through the house, is harder. A roof above a second storey may need scaffold, and a heavy unit may need lifting gear.
Some Milton Keynes houses sit close together, and some rear gardens can be reached only through the property. We note access at the survey and plan the day around it.
Allow one or two days on site for the fitting of a standard unit. A bigger or more awkward opening takes longer, and we say so before we start.
The finish inside the room
The inside of the opening is the part you will look at every day. A plain square reveal is the simplest finish. A deep, splayed lining that throws light wider into the room is more work, and it shows. Plastering, decorating and making good the ceiling around the opening are all part of getting it right.
We finish the inside, so the work is tidy when we leave. That includes the lining and the plaster return, not only the glass. If you want a particular decorative finish beyond that, tell us at the survey and we will price it into the same fixed quote.
A deeper reveal also changes how the light behaves. A square shaft keeps the beam narrow and bright. A splayed one spreads it. Both are valid, and the choice is aesthetic as much as practical.
Words you will hear in a quote
Upstand
The short insulated kerb on the roof that the rooflight glass sits on.
Fall
The slight slope of the glass and roof that sends rainwater off instead of letting it stand.
Trimmer
A timber that frames the opening in the roof structure so the cut joists are still supported.
Membrane
The waterproof roof covering that wraps the upstand and seals it to the roof.
Reveal
The lined sides of the opening between the rooflight and the ceiling.
Laminated glass
Glass with a bonded inner layer that holds together if the outer pane breaks.
Rules that shape the specification
Fitting a rooflight in an existing house is building work that has to meet current Building Regulations. If the job has to be notified, we deal with Building Control for you, through the council or a registered building control approver. We install to current regulations, and the heat loss figure above is one reason the glass specification is not a free choice.
Planning is a separate question. Some rooflights fit within permitted development, but your own situation depends on the property and its location. Ask the council's planners whether your street lies within a conservation area or has an Article 4 direction. The rules in full are in our planning guide.
England sets no daylight rule for houses under the Building Regulations, so the size of the opening is your decision, not a figure set by law. That freedom is worth using well.
Getting a price you can rely on
Cost is only worth comparing when two quotes describe the same job. Before you compare, check that each one names the same size, the same opening type, the same glass, the same upstand build, and says who makes good the inside.
Our process is short. We visit, measure, look at the roof and talk through the choices. Then we send a fixed written quote that does not change on the day. The survey is free, and so is the conversation about options, and the 10-year workmanship guarantee comes in writing.
For a small extension where a flat rooflight is up against a lantern, our answer on the small-extension choice sets out the trade-offs. If you are still checking what is possible, see whether a flat roof can take a skylight, and when you are ready, return to the flat roof rooflights overview.
Before you ask for a quote
- Where in the room should the sky show?
- Does it need to open, or is light enough?
- Is the roof covering sound around the opening?
- How will the unit reach the roof?
- What finish do you want inside the opening?
Cost questions, answered plainly
Why will you not give a price over the phone?
A flat rooflight is made to your roof, and the price follows the size, the glazing, the kerb and the state of the roof beneath. Before we have measured, any figure would be a guess. The free survey ends with a fixed written quote that holds on the day.
Does a bigger rooflight always cost more to fit?
The glass costs more, and so can the structural work if the opening is wider than the joists allow. A well-placed smaller unit can do more for a room than a larger one in the wrong spot, so size to the part of the room you use.
Is the upstand included in the quote?
Yes. The upstand, its insulation and the waterproofing that wraps it are part of the job, and they are written into the fixed quote with the glass and the finish inside.
