Why a rooflight counts as building work
Rooflight building regulations catch people out because a rooflight looks like a fitting, a bit like a new radiator, when the law treats it as part of the building's envelope. Cutting a hole in a roof changes how the roof carries load, how heat escapes, how fire could spread and how safe the glass is. Skylight building regs cover all four, and they apply to a new rooflight, a bigger one and a replacement alike.
In England, putting in a new rooflight, or exchanging an old one for a new one, is controlled work as far as the Building Regulations 2010 are concerned. That does not make every job a major one. It means the work has to meet the standards in the regulations, and the right person has to be told about it. The standards themselves sit in the Approved Documents, which are the government's published guidance on how to comply. They are not law in themselves, but following them is normally taken as showing that the work complies.
This page is the overview. It explains which parts apply to a rooflight, what the headline numbers are, and who checks the work. The details of choosing glazing sit in our energy efficient rooflights section, and permission to build sits in a separate system, covered in our planning permission answer.
Planning and Building Regulations are easy to mix up. Planning asks whether you may build something where you want it. Building Regulations ask whether it is built safely and to a proper standard. Some rooflights need one, some need both and some need neither, and having one never excuses you from the other.
Structure: when cutting a rafter becomes notifiable
The first question a surveyor asks is whether the new opening interrupts any structural timber. A roof is a frame, and rafters, joists and purlins all carry load into the walls. A rooflight that sits entirely between two rafters, with no timber cut, leaves the frame as it was. Cut a rafter to make room, and you have taken a part of the frame out. That is structural, and it is the point at which the work needs proper calculation and sign-off.
Flat roofs behave the same way. Joists run across the span, and a large opening for a flat rooflight or a roof lantern can interrupt several of them. The loads from the cut joists are picked up by trimmers, which are doubled-up timbers around the opening, and by the joists either side. The size of those timbers depends on the span, the spacing, and the weight of whatever goes on top. A heavy glass lantern puts more load in than a small acrylic dome, and a walk-on panel more again.
Older houses add a wrinkle. Victorian terraces often have hand-cut rafters at uneven spacings, and some roofs have been altered over the years in ways that are not obvious from inside the loft. Newer houses frequently have trussed rafters, which are engineered as a unit and should not be cut without the truss maker's or an engineer's design. We check which you have before we propose a size, so the quote reflects the real structure.
Where the structure is altered, the design is set out on a drawing or calculation and passed to the building control body before work starts. That is the main reason structural jobs are notified in advance rather than after the event. If the roof is not touched structurally, the notification is simpler, but a replacement still falls under the same regulations.
Cutting a rafter or a joist is where a rooflight stops being a fitting and becomes structural work.
Heat loss and the Part L figures
Part L of the Building Regulations covers the conservation of fuel and power. For a rooflight it sets how much heat the unit may let through, measured as a U-value in watts per square metre per kelvin (W/m²K). A lower figure means less heat lost.
England's existing-dwelling limits for new and replacement units live in Table 4.2 of the 2021 Approved Document L, volume 1. A roof window needs 1.4 W/m²K or Window Energy Rating band B, and a rooflight needs 2.2 W/m²K. Roof lanterns count as rooflights. The distinction matters because a roof window sits in a pitched roof and is made like a window, while a rooflight is a broader term for glazing in a flat or low-pitched roof, and the allowance is more generous.
A newer edition of Approved Document L for England comes into force on 24 March 2027. For existing dwellings it keeps the same two figures, and the table is renumbered as Table 3.2. Until then, the 2021 edition and Table 4.2 apply.
Wales has its own Approved Document L, the 2022 edition, which applies to work in Wales, and a 2026 edition follows from 4 March 2027. If you are in Flintshire or Wrexham, the figures to check are the Welsh ones. The local authority building control team there will confirm which apply.
Most good modern double-glazed units meet these limits, so the figures rarely block a job. They matter when someone offers a cheap unit with a thin sealed pane, or an old unit is being kept in a new extension. The label or the product data sheet should state the U-value or the WER band, and we check it before ordering. The figures are worked through in U-values and Part L for rooflights.
| Unit | Limit | Source |
|---|---|---|
| Roof window | 1.4 W/m²K, or WER band B | ADL 2021, Table 4.2 |
| Rooflight | 2.2 W/m²K | ADL 2021, Table 4.2 |
| Roof lantern | 2.2 W/m²K | Counts as a rooflight |
Fire: boundaries and escape
Part B of the Building Regulations deals with fire safety, and two parts of it touch rooflights.
The first is the spread of fire across roofs. Where a roof is close to a boundary, the roof covering and any rooflight in it have to resist fire spreading from one building to the next. The rules differ for glass and plastic, and they tighten as the distance to the boundary shrinks. On a terrace, the neighbour's roof is attached to yours, so the point is to check which classification the rooflight carries and whether it suits where it sits. That check belongs in the survey, not on site halfway through the job.
The second is escape. A rooflight is not normally an escape route, and on most houses nothing in the regulations asks it to be. The exception is a room in the roof of a bungalow or chalet bungalow, where a roof window within reach of the ground can serve as an escape window if it opens far enough. Approved Document B, volume 1, gives the sizes. The opening must be at least 0.33m² unobstructed, at least 450mm high and 450mm wide, and its bottom edge no more than 1100mm above the floor. A roof window cannot be the escape route from the converted loft of a two-storey house, because the 4.5m height limit for escape windows is passed; a protected stair is needed there. Can a roof window be a fire escape window? takes the question step by step.
Safety glass and what sits overhead
Glass in a roof has two risks that glass in a wall does not. It can break and fall onto people below, and in some cases people can stand or fall onto it from above. The regulations and good practice both address this.
For glass above head height, the usual answer is a laminated inner pane. Laminated glass holds together when it cracks, because a plastic interlayer sticks the fragments in place, so a broken pane stays in the frame rather than dropping into the room. Roof windows and rooflights are commonly supplied with a laminated inner pane for this reason.
Walk-on glass is a different matter. Glass that people can step on has to be designed for the load and the use, and the manufacturer must rate it for foot traffic. We do not substitute an ordinary rooflight for a walk-on one, whatever the opening looks like. The same care applies on a flat roof that is used as a terrace, where the glass may be walked on even if it was never meant to be. Our guide to walk-on rooflight load ratings sets out what the ratings mean.
For anyone working on a roof, the fragility of old rooflights is a hazard in itself. Plastic domes and wired glass that have aged can give way underfoot. It is one more reason the work is done by people used to roofs, with the right access and edge protection.
Who checks the work
In England, you have a choice of who inspects. One route is the local authority building control, which is the council's own team, such as Liverpool City Council, Wirral Council or Sefton Council. The other route is a registered building control approver, the name for a private inspector since April 2024. Either can be used for a rooflight.
The notification goes in before the work starts. If the job is notifiable, we deal with the notification to Building Control on your behalf. The inspector may ask for drawings, product data or photographs, or visit the site, depending on the size of the job. Once the work is signed off, the council gives you a completion certificate, and a registered approver gives you a final certificate. That is the paper a solicitor asks for when you sell.
In Wales, the Building Regulations 2010 apply as they apply in Wales, with the Welsh Approved Documents. Welsh building control comes from the local authority, which here means Flintshire County Council or Wrexham County Borough Council, or from a registered building control approver, as Wales has had registered approvers since April 2024 too.
We do not ask you to arrange any of this. What we do ask is that you tell us early if you are planning the rooflight as part of a larger job, such as an extension or loft conversion. In that case the rooflight is covered by the main building control application, and the inspector sees it as part of the whole.
Skipping the notification has a cost later. A council can ask for work to be opened up for inspection, and a missing certificate can slow a sale. Our guide to replacing a rooflight shows where the notification fits in the job.
What the regulations do not ask for
One misunderstanding is worth clearing up. The Building Regulations set no daylight requirement for houses in England. There is no minimum glazed area, no rule that a room must have a window of a given size, and nothing that says a dark landing must be lit. Daylight standards exist, but they are guidance for designers and for planning, not requirements for a house.
That means the size and number of rooflights is your choice, subject to the limits above on heat loss, structure and safety. A bigger rooflight is not required to pass, and a smaller one is not penalised. The practical limits come from the roof itself, what the rafters or joists allow, and the budget.
It also means the regulations care about what a rooflight does for ventilation, heat and safety, not how much light it lets in. If you are unsure which rules apply to a particular room, the survey covers it.
Pricing any of this starts with a free quote, and what it costs to replace a skylight sets out the factors behind the figure. The main skylight and rooflight installation page shows all ten services.
Common points at the survey
Is swapping one rooflight for another notifiable?
In England, a replacement rooflight is controlled work, so yes, the Building Control notification applies. If it is notifiable, we take care of it.
Do the Part L limits apply to a unit I am keeping?
The limits apply to new and replacement units. A rooflight you leave alone is not brought up to the current figures, although it may be worth replacing if it is single-glazed.
Does a rooflight in a flat have different rules?
The Building Regulations apply to flats as well. Planning permitted development rights are different for flats, and that question is answered separately in our planning guide.
What certificate do I get at the end?
A completion certificate if the council inspected, or a final certificate if a registered approver did. Both are kept with the house papers.