Short answer
There is no set number. How many rooflights an extension needs depends on the shape of the room, the direction of the joists and where the daylight is wanted. A long, narrow kitchen often suits one large unit or a row of equal ones. A square room may suit a single big rooflight or a lantern. No Building Regulations daylight rule for houses fixes the figure, so the choice is a design one, and the survey lays it out on the plan.
There is no rule for how many rooflights an extension needs
People often ask for a figure, such as one rooflight for every so many square metres of floor. No such rule exists for houses in England. The Building Regulations have no daylight requirement for dwellings, and Part O, which deals with overheating, applies to new dwellings only. What the regulations do cover is how well each unit keeps heat in, and the structure that carries it.
So the number of rooflights for an extension is decided by the room, not by a formula. We start from how the space is used, where people sit and cook, and which way the joists run, and then test a few layouts on the survey sheet. The written job plan records the one you choose.
One large unit or several smaller ones
A single large rooflight gives one clean opening in the ceiling and one kerb on the roof, which also means one set of flashings to detail and one membrane edge to finish well. It suits a wide room, because the glass spreads across it evenly, and it keeps the roof simple. The trade-off is weight and handling: a big unit is heavy to lift, and the opening needs a strong header across it.
Several smaller units suit a long or narrow room better. In a kitchen extension, the number of skylights usually follows the length of the room. A row of three, evenly spaced, follows the line of the kitchen and keeps each opening within the span of the joists. The joists between them stay intact, which can lower the structural work. Smaller units are also easier to carry up and set, though each needs its own kerb and its own piece of membrane. To see how they look from inside, read about frameless flat rooflights. Frameless units have a slim edge, so several in a row read as a calm band of glass.
| Layout | Where it tends to suit |
|---|---|
| One large unit | Wide, open rooms; a clean single opening |
| A row of equal units | Long, narrow rooms; follows the kitchen run |
| Two units, different sizes | A room split into cooking and eating zones |
| A lantern | A square room where a central focus is wanted |
Joist direction sets the layout
The joists run one way across the roof, and that direction shapes what is easy. An opening that runs along the joists, between a pair of them, needs little extra framing. An opening that runs across them has to cut one or more, and the load they carried is picked up by a header and trimmers.
For that reason the structure gets checked before any layout is drawn. If the extension is new, the joists can be set out to suit the rooflights. If it is already built, we work with what is there, and the plan shows which openings are straightforward and which need more framing. The choice stays yours, with the cost and the work explained.
Rooms that are already built often raise a different question: can one more unit be added later? Usually it can, provided the joist run allows it and the new opening is kept clear of the others. It is cheaper to plan a second kerb alongside the first, so if you think you may want another, say so at the survey and the plan can leave room for it.
Over the kitchen run or over the table
Where the glass sits changes how the room feels. A rooflight over the worktops gives good light for preparing food. One over the dining table lights the place where people sit for longer. Over an island, it does both, and our photo list shows a frameless unit above a kitchen island for just that reason.
Some rooms do best with a split. A smaller unit over the cooking side and a larger one over the table gives each zone its own light, with a plain stretch of ceiling between them for downlights or an extractor. Think about where the light fittings, cooker hood and any bi-fold doors sit, because those compete for the same ceiling. Mark them on the plan before the openings are fixed, since moving a kerb later costs far more than moving a pendant light, and a few minutes with a pencil settles most of these questions.
- Over the worktops: steady light for preparing food
- Over the table: light where people sit for longer
- Over an island: covers both in one opening
- Between: plain ceiling kept for downlights and extraction
How the survey lays it out
During the free survey, the room is measured, joist direction and spacing are noted, and two or three layouts are drawn up. Each shows the unit sizes, the kerb positions and the structural work involved. You see them side by side, on paper, before you decide.
The sizes themselves are covered in flat rooflight sizes, and timing with the builder is covered in when the rooflight goes in on a new extension. Each layout carries its own written quote, so the effect of a second unit on the total is clear, and what drives flat rooflight cost explains the reasons behind it.
What the job plan records
Once you choose, the job plan from Ridgeway Skylights lists each unit with its size and marks the openings on a sketch. It also notes where permission stands at your address, who notifies Building Control, the expected glass delivery and the fitting date. Our 10-year workmanship guarantee applies to the finished work.
The permission position is checked for your address. Most extensions with rooflights fall under permitted development, but a planning condition on the house can change that, and the council decides. For the broader options, the guide to flat roof rooflight installation describes each type.