Cut an opening in a roof, set glass into it, and the Building Regulations have something to say. The Welsh rules are not the English ones with the names changed. Wales publishes its own Approved Documents, glazing safety sits in a different document, and the Welsh tables are numbered differently.
A new skylight, rooflight or roof window in an existing Welsh home meets several parts of the regulations, and they arrive in a sensible order: whether the job counts as building work, who checks it, how well the glass holds heat, how the room breathes, how a loft bedroom empties in a fire, and what keeps people safe around the glass. Planning permission is a separate question, and our planning guide for Wales answers it.
A skylight is a controlled fitting
Under the Building Regulations 2010, fitting any of the three, a window, a roof window or a rooflight, counts as building work. The label that does it is "controlled fitting": a fitting that some technical part of the regulations sets a requirement for. Approved Document L (Wales) lists all three of these by name.
A brand-new opening and a swap are treated differently. According to the Welsh Government, approval will generally be needed when a rooflight is new, and it lists four grounds. Making the opening usually involves changes to the roof structure. The unit's weight has to be within what the roof can carry, or the roof gets strengthened. Energy efficiency counts. And if the unit sits near a boundary, its fire performance is part of the picture. Together they explain why a new hole in a roof draws more attention than a like-for-like swap.
Swapping a window or roof window for another of its kind is building work too, while repairing one is not. If only the glass changes and the frame stays, the energy rules do not treat it as building work. The gov.wales note on rooflights is the source for the list of four.
- Controlled fitting
- A fitting that a technical part of the Building Regulations places a requirement on. Approved Document L (Wales) names windows, roof windows and rooflights among them.
- Roof window
- A window that faces the same way as the surrounding roof and lies in its plane.
- Rooflight
- A glazed unit raised on a kerb or upstand rather than lying in the roof plane. The document adds that it is sometimes called a skylight.
Who checks the work, and who tells them
Two kinds of body can check the work, and you pick between them. One is the local authority's building control service. The other is a private registered building control approver. A private firm has to be on the Building Safety Regulator's register, and the inspecting is done by registered building inspectors, or on their advice. The council's own service sits under different oversight: the Welsh Ministers regulate it, not the Building Safety Regulator. The Welsh Government sets the arrangement out on its page about regulation of the building control profession.
The registered approver system began in Wales on 6 April 2024. The older approved inspector route is no longer a current option, since approved inspectors who had not registered could carry on with ordinary building work only until 1 October 2024. The Planning Portal's Welsh pages describe where to get approval.
A like-for-like replacement has a third route. In an existing home, an installer registered with a scheme the Welsh Government has authorised can self-certify a replacement of this kind: window, door, roof window or rooflight. The installer tells the local authority and issues a certificate of compliance. The schemes are listed on gov.wales, and they are for replacements. A brand-new rooflight is different, and it will generally need approval from a building control body.
Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver. Finding the right office is not left to you.
In Wales the choice of building control body is yours: the council's own service or a registered building control approver.
How well the glass has to hold heat
Approved Document L (Wales), Volume 1, deals with energy in dwellings. The edition in force is the 2022 one with 2024 amendments, in effect since 23 November 2022. A 2026 version takes over on 4 March 2027, and the numbers for windows, roof windows and rooflights are unchanged in it.
The document is guidance, so it speaks in terms of what a product should meet. For an existing home, a roof window, whether new or replacement, should reach 1.4 W/m²K or Window Energy Rating Band B. A rooflight, new or replacement, should reach 2.2 W/m²K. England uses the same two figures. The Welsh tables carrying them are Table 4.1 and Table 10.1.
| Element | Value it should meet | What the document means by it |
|---|---|---|
| Roof window | 1.4 W/m²K, or Window Energy Rating Band B | Flush with the slope around it |
| Rooflight | 2.2 W/m²K | Glass unit on a kerb or upstand, out of the roof plane |
Reading a U-value properly
A U-value measures heat loss, so a lower number means less heat gets out. Rooflight U-values count the unit's whole developed surface, and that area can exceed the hole in the roof. When the kerb ships as part of the rooflight assembly, the 2.2 limit applies to the kerb too.
The document defines a rooflight by its kerb or upstand, and roof lanterns are not mentioned by name. For that reason we keep the rooflight figure out of any claim about lanterns. Our page on roof lanterns, planning and Building Regulations takes that product on its own.
One fallback exists. Suppose a replacement window or roof window sits in a house of historic or architectural interest and cannot reach 1.4. The document then allows 1.2 W/m²K at the centre of the pane, or low-e secondary glazing. Rooflights and new openings get no such fallback.
Fit matters too. Once installed, the glass units should sit snugly and keep draughts to a reasonable minimum. On glazing choice, VELUX describes double or triple glazing on its flat and curved glass rooflights, aimed at energy efficiency and less noise. Our guide to Part L and rooflights in Wales goes deeper on the energy side.
Fresh air: ventilation after a window changes
Approved Document F (Wales) handles ventilation. Its 2022 edition took effect on 23 November 2022, and a 2026 edition follows on 4 March 2027 with the replacement-window paragraph reading the same in substance. After dark the glass in a roof reads as black and shows the room back at you, so it is easy to forget that the window may also be a ventilation path.
The starting point is simple. A replaced window should leave the home's ventilation no worse than before. Where the old window carried trickle vents, also called background vents, the new one should carry vents no smaller than the ones it replaces, controllable by whoever lives there or automatically. The exception is a home with continuous mechanical supply and extract ventilation.
Where the old window had no trickle vents and the house relies on natural ventilation, one way to show ventilation is no worse is to fit vents of a stated minimum size. If those sizes are not technically feasible, the vents should come as close as feasible. The bathroom figure is the smaller one. Background ventilators should sit at least 1.7 m above floor level.
Purge ventilation is a different idea: a way to air a room quickly. Every habitable room needs it, and an opening window can supply it. For a hinged or pivot window that swings out 30 degrees or wider, the opening area should come to a twentieth of the floor area; if it opens between 15 and 30 degrees, a tenth is needed. Anything opening under 15 degrees is left out of the count. The document does not mention rooflights specifically for kitchens or bathrooms. The Welsh Government publishes it on its Approved Document F page.
Approved Document F (Wales): the numbers behind the wording
- 8,000 mm²equivalent vent area in a habitable room or kitchen, one way to show ventilation is no worse where the old window had no trickle vents
- 4,000 mm²equivalent vent area in a bathroom, on the same basis
- 1/20opening area to floor area for a hinged or pivot window opening 30 degrees or more
- 1/10opening area to floor area where the window opens between 15 and 30 degrees
Escape from a loft bedroom
Approved Document B (Wales), Volume 1, covers fire safety in houses. It is the 2006 edition with amendments to 2020, so the paragraph numbers from England's 2019 edition do not carry across. For a roof window, one relevant part is the emergency escape window.
An escape window must have an unobstructed opening of at least 0.33 m², no less than 450 mm high and 450 mm wide, with the bottom edge of the opening no more than 1,100 mm above the floor. Through it, a person must be able to reach somewhere safe from the fire. The sash should hold itself open unaided. Locks and stays can be fitted, as long as the stay has a release catch. In a roof window the bottom of the opening may be 600 mm above the floor, where the usual guarding height is 800 mm.
The document ties all this to height. For upper-floor habitable rooms where the floor is not more than 4.5 m above the ground, the route out is an escape window or direct access to a protected stairway. A house with a floor higher than 4.5 m needs a protected stairway, or the top storey separated with its own alternative escape route, unless it has more than one separated internal stair. Making a room in the roof space of a two-storey house would normally call for fire-resisting doors and partitions around the stairway all the way down, where none stood before.
Replacing a window in a spot where a new house would need an escape window raises a further point. If the old window was big enough to escape through, the new opening should give at least the same escape potential. If the old one was larger than it needed to be, it can come down to the minimum above. Wales adds a rule of its own: where a house already has an automatic sprinkler system, the system should be extended and re-certified to cover altered, extended and added areas. The Approved Document B page holds the full text.
Safety around the glass: Approved Document N
Wales keeps a document of its own for glazing safety, Approved Document N, covering impact, opening and cleaning. The Welsh version is the 1998 edition with 2000 and 2010 amendments.
Its central idea is the critical location. Glazing in walls and partitions, inside and out, counts as critical from the floor up to 800 mm. In a door, or in a side panel close to either edge of a door, the zone runs up to 1,500 mm. Glass there should break safely, be strong enough or divided into small panes, or be permanently protected. N3, the requirement on opening and closing windows and skylights safely, does not apply to dwellings.
The Approved Document N page links the full document.
What to settle before the job
Behind the work sits a handful of short questions. Is the opening new or a replacement? Is the room a loft bedroom that needs an escape window? Does the old window carry trickle vents? Each answer decides which paragraph above applies.
We begin with a free survey before any price, and what follows is a fixed written quote. A 10-year workmanship guarantee, in writing, comes with the work. For a new or replacement skylight, rooflight or roof window, ask for a survey and we will confirm which route the job takes. Our services page shows everything we fit, while does a roof window need Building Regulations and VELUX and Building Regulations in Wales take two products in turn.