Cutting a roof window into a pitched roof means cutting rafters, and that is why the Building Regulations take an interest. The opening changes the structure, the glass changes how much heat the room loses, and in a loft bedroom the window may be the way out in a fire.
In Wales those points are covered by the Welsh Government's own Approved Documents, each marked for use in Wales, and several of the paragraph numbers and table references differ from the English editions that turn up in most search results. The sections below follow the Welsh versions.
Which of them bite on your roof depends on the room and on whether the window is new or a replacement. We sort that out at the free survey, before a fixed written quote.
What counts as building work
Under the Regulations a roof window ranks as a controlled fitting, and installing one is building work. A fresh opening in a pitched roof is the clearest case. The Welsh Government's roof lights page sets out why approval is generally required for a new rooflight, and its grounds fit a roof window cut into a slope as well as a flat unit. Cutting the opening usually disturbs the roof structure. The roof must be able to take the unit's load, with strengthening if it cannot. The unit must be energy efficient. Near a boundary, fire performance enters the picture too.
Every product type is covered in the wider guide to skylight Building Regulations in Wales. Replacement is a separate question. Replacing one roof window with another is itself building work, size for size included. A repair stays outside the Regulations. Fresh glazing in a frame that remains is also left out of building work by the energy rules.
Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver.
| Job | Building work? | Why |
|---|---|---|
| New roof window, new opening | yes, approval generally needed | the roof structure is usually altered |
| New roof window in place of an old one | yes | replacement counts |
| Repair to an existing window | no | repairs are outside the Regulations |
| New glass, existing frame kept | not for the energy rules | the frame stays |
Who checks the work
Wales has two kinds of building control body. One is the local authority building control service; the other is a private registered building control approver. Customers are free to choose which they use on a project, and the Planning Portal's comparison of building control bodies sets out the difference.
The two are overseen differently. Private firms must be registered with the Building Safety Regulator as registered building control approvers, and their checks are carried out by, or on the advice of, registered building inspectors. Local authority building control is regulated by the Welsh Ministers. The Welsh Government explains the split on its regulation of the building control profession page.
That regime started on 6 April 2024.
Energy: 1.4 and the glazing total
Approved Document L (Wales) gives the figure for the glass. A new or replacement roof window in an existing home should have a U-value no worse than 1.4 W/m²K, or a Window Energy Rating of Band B. Either route is open to a new or replacement roof window in an existing home, so a quotation should say which one the window is meeting.
A roof window sits in the plane of the roof around it. A rooflight, which stands on a kerb or upstand, takes the higher figure of 2.2 W/m²K, so check which product a quote describes before comparing numbers. The second figure is covered on our Part L in Wales page.
Older homes get one concession. Where a replacement window or roof window on a home of architectural or historic interest, or on a façade whose character must be kept, cannot reach 1.4, the fallback is 1.2 W/m²K at the centre of the pane, or low-e secondary glazing. For a conservation roof the conservation rooflights page explains the products.
There is also an area test. Under Approved Document L (Wales), when a window or door opening is enlarged or a new one made, the total area of windows and doors should not exceed 25% of the home's total floor area. From 4 March 2027 the new Approved Document L (Wales) names roof windows and rooflights in that total, and allows more than 25% if the home's energy efficiency is improved to compensate. The roof window value of 1.4 W/m²K is unchanged in that edition. Fitting should be tidy and fairly draught-free.
Roof window energy figures, Wales
- 1.4W/m²K, roof window
- Band BWindow Energy Rating alternative
- 1.2W/m²K, fallback for historic homes
A loft room and the way out
Approved Document B (Wales) deals with fire safety in dwellings, and it matters most when a roof window goes into a loft bedroom. Its edition for Wales is the 2006 one with later amendments, so English paragraph numbers do not carry over. An escape window must have an unobstructed opening of at least 0.33 m², at least 450 mm high and 450 mm wide, with the bottom of the opening no more than 1,100 mm above the floor. It must also let the person reach a place free from danger from fire.
Two practical points follow. The sash has to hold itself open, and a lock or stay is fine provided the stay has a release catch. And in a roof window, the bottom of the opening may sit 600 mm above the floor, where the usual guarding height is 800 mm.
A roof window provides escape only if it has been sized and positioned for that job. The size, the pitch and the height above the floor all have a say, which is why we look at the room first.
Floors above 4.5 m and loft conversions
An escape window, or direct access to a protected stairway, is the route out of upper-floor habitable rooms where the floor is not more than 4.5 m above ground level. A house with a floor higher than that needs a protected stairway, or its top storey separated with its own alternative escape route, unless it has more than one separated internal stair. A loft conversion on a house of two storeys will normally mean enclosing the staircase in fire-resisting doors and partitions from top to bottom, where there were none. One Wales-only point: existing sprinklers or comparable automatic suppression are to be extended, then certified again, across any altered or added area.
Replacing a window that already serves as an exit
If a replacement goes where a new build would call for an escape window, and the old window was big enough to escape through, the new opening should offer the same escape potential at least. Where the old one exceeded the minimum, it may be cut down to it. The Welsh Government's Approved Document B page has the document.
Escape window, at a glance
- 0.33 m²minimum clear opening
- 450 mmminimum height and width
- 1,100 mmhighest bottom edge above floor
Ventilation, steam and glazing safety
Under Approved Document F (Wales), replacing a window should leave the ventilation no worse than it was. Trickle vents on the old window mean vents on the new one, no smaller, which either the occupant or an automatic mechanism controls; the one exception is a home with continuous mechanical supply and extract. With no vents on the old window and a house that relies on natural ventilation, one route is background vents. Habitable rooms and kitchens get at least 8,000 mm² of equivalent area on that route, bathrooms at least 4,000 mm², and the vents go as near those sizes as feasible where the full size cannot be fitted.
Each habitable room also needs a way to air it quickly, called purge ventilation, and an opening window is one way to provide it. A hinged or pivot window that opens 30° or further qualifies when its opening area is a twentieth of the floor area, while an opening angle from 15° up to 30° calls for a tenth. Under 15° it does not count. How each type opens is explained in the guide on how a roof window opens.
Glazing safety is a separate document. In Wales it is Approved Document N, titled for impact, opening and cleaning. Guarding against a fall from a height over 600 mm belongs to the Welsh Approved Document K.
Planning is a separate yes or no
The Building Regulations never answer the planning question. In Wales, adding a rooflight to a house in a conservation area, in Bannau Brycheiniog National Park, inside the Gower AONB or in a World Heritage Site is not permitted development, so it needs a planning application. Elsewhere, a roof window on a house may stand no more than 150 mm beyond the plane of the original roof slope, and no higher than the highest part of the original roof.
A window on a side-facing slope that, projected down to the ground, would sit within 10.5 m of the boundary opposite that side must be obscure-glazed, and fixed shut unless the opening part is at least 1.7 m above the floor or stair directly below it. Householder rights of this kind are not available for a flat or maisonette, and an Article 4 direction can remove them from a house. On a listed building, a roof window that affects its character needs listed building consent, applied for through the local planning authority.
Our guide to roof windows and planning permission in Wales takes those rules in order, and the skylight planning guide sets out the full conditions. The free survey is the point at which we confirm where your house stands.