A new roof window looks like a small job from the garden. From the roof structure's point of view it is a hole in the frame. Rafters are cut and supported, a new opening takes load, and heat leaves through the glass. The Building Regulations exist to make sure that hole is made safely and does not waste energy.
The rules are often confused with planning permission. They are separate systems. Planning decides whether you may change the look of a building, and for roof windows on houses that is usually answered by permitted development. Building Regulations decide whether the work is built to a standard, and they apply whether or not planning is needed.
This guide takes the main requirements in turn: the structure around the opening, the energy figures for the window, the point where a window in a loft room becomes part of the escape route in a fire, and the practical question of who must be told. The figures come from Approved Documents A, B, C and L and from the Planning Portal.
Nothing here replaces the view of your building control body, whose decision is the one that counts. The aim is to leave you knowing which questions to ask.
Which parts of the Regulations a roof window touches
The Building Regulations are organised into parts, each covered by an approved document. Four of them matter for a new roof window.
Structure
Part A deals with structural safety. A new opening in a roof means cutting rafters, and the loads they carried have to be carried some other way. Approved Document A sets the idea of a significant change in loading, which it treats as an increase of more than 15 per cent, and a new opening is a change to the structure that building control will want to see handled properly.
Fire and escape
Part B covers fire safety. It matters most when the window serves a habitable loft room, because it may need to work as an escape window. Section 4 of this guide looks at that.
Moisture
Part C wants a roof to keep rain out of the building and to stay sound and well insulated despite hidden condensation inside the construction (paragraph 6.2). A badly flashed window, or one set in a gap in the insulation, falls short of that.
Energy
Part L covers energy use and sets the figure the window itself must reach.
Each of these applies to the work, whether or not an application is needed, and they apply to the person doing the work as much as to you. The next sections take them one at a time, beginning with the structural opening.
Our roof windows and skylights page covers the full range of work, and the condensation and leak guides cover the aftercare side.
Trimming rafters and supporting the opening
Rafters are the sloping timbers running up from the eaves. A new window occupies the space between them, and often more than one rafter's width.
What trimming means
If the opening fits between two rafters, the job is lighter: a header and a sill, short cross-pieces, are fixed between the rafters to frame the opening. If the window is wider than the gap, one or more rafters must be cut. The cut ends are then carried on a trimmer, a doubled timber fixed across the opening, and the load passes to the rafters on either side, which have to be strong enough to take it.
What this does to the roof
The rafters either side of the opening now carry extra. The trimmers and headers have to be sized for it, and the fixings made properly. A window placed carelessly, with a rafter cut and left unsupported, can sag the roof.
Who checks
Building control looks at the structural design of the opening. For larger openings, or where the roof structure is unusual, a structural engineer's calculation may be asked for, and the installer should say so before the roof is opened. In the Test Valley area, the body to ask is local authority building control or a registered building control approver.
Position matters
Lining the window up with the existing rafter spacing saves work and strengthens the result. Keeping clear of valleys, hips, the ridge and chimneys avoids awkward framing.
For a like-for-like swap, with the opening unchanged, none of this applies, because the frame stays where it is. A new opening is a different matter, and it is the one this guide is about.
The energy figures for windows and rooflights
Part L is where the common mistakes are made, so the figures are worth setting out carefully.
Roof windows
Approved Document L, Volume 1, in the 2021 edition, Table 4.2, asks a new or replacement window in an existing dwelling to reach 1.4 W/(m2K), or Window Energy Rating Band B at the least. A note on the window row says roof windows and curtain walling are included.
Rooflights
A rooflight, a glazed unit in a flat or low-pitch roof, has its own figure of 2.2. That figure is for rooflights alone. Roof windows are held to the window figure above, and mixing the two up is a common slip in quotes and articles.
A note on terms
Band B is one rating on the Window Energy Rating scale. Treat it as a named standard and read it as stated, without guessing where it sits in the ranking.
The surrounding roof
The regulations treat walls, floors and roofs as thermal elements and leave glazed and door units out of that definition. The window is judged by its own figure, and the roof around it as a roof. The insulation round the frame has to be continuous, or heat escapes at the edges.
Editions
The 2021 edition runs until 24 March 2027, and then the 2026 edition takes over in England under new table numbers, 3.2 and 3.3.
What to ask for
The supplier's data sheet states the window's U-value or its energy rating. Keep a copy, because building control may ask to see it.
Escape windows in loft rooms
Fire safety is the part of the regulations that most affects a window in a room where people sleep.
When it applies
It applies where a window serves a habitable room in the roof, such as a bedroom in a loft conversion. A window in a landing or a store is a different question. The escape rules are in Approved Document B, and building control applies them to your plans.
What an escape window must be
Paragraph 2.10 asks for a clear openable area of at least 0.33 square metres, with a minimum height and width of 450 mm each. The bottom of the openable area must be no more than 1100 mm above the floor. Those figures are minimums, not targets, and they apply to the clear opening when the window is open.
The 4.5 metre limit
An escape window serves only storeys within 4.5 metres of the ground, since beyond that height nobody can realistically climb out or be reached from outside. A tall house with a second floor loft may have a storey above that limit. Paragraph 2.5 deals with that case: a storey more than 4.5 metres above ground needs a protected stairway or an alternative escape route.
No "always"
On a two-storey house the usual answer is a protected stair, though AD B allows alternatives, among them sprinklers in the open-plan areas (paragraph 2.23). Which route suits your house is for building control to say.
Our guide to roof windows for a loft conversion goes into the placement and the escape questions in more detail.
For any bedroom window, check the size before you order, not after the roof is cut.
Planning and building control are separate
It is easy to assume that if planning permission is not needed, nothing else is. The reverse is the safer assumption.
Planning
For a house, Class C of the permitted development order normally covers a roof window. The conditions include a 0.15 metre limit on how far the window projects from the original roof slope, and obscure glazing for a window in a side slope. A flat has no permitted development rights. Conservation areas, listed buildings and Article 4 directions can change the position. See our guide to conservation roof windows.
Building Regulations
These apply to the work whatever the planning answer. The Planning Portal's thresholds for a re-roof, under 25 per cent of the building envelope and under 50 per cent of the roof, refer to re-covering. They do not excuse a new structural opening. The envelope includes roof windows and rooflights, which shows how seriously the regulations treat them.
Notifying
Find out from building control whether the work goes in by building notice or full plans. Registered approvers offer a private route alongside the council. In Test Valley, do not assume either: ask.
Competent person routes
Some installers can self-certify certain work, but the rules differ by trade and the route should be confirmed with the installer before the job.
Party walls
Where the new window touches a shared wall, the question goes to a party wall surveyor and not to building control.
Keep the paperwork. A future buyer's solicitor is likely to ask for it, and gaps in the paperwork can delay a sale.
What to ask before the roof is cut
A short list of questions, put to the installer and to building control, heads off problems.
For the installer
- What size is the opening, and does it fit between the rafters?
- If a rafter is cut, how is it supported, and who has designed that?
- Which window figure does the product meet, and can I see the data sheet?
- Which flashing kit suits my covering?
- Who notifies building control, and what certificate follows?
For building control
- Does this need a building notice or a full plans application?
- Does the structure need an engineer's calculation?
- Is the window serving a room that needs escape provision?
- Which edition of Approved Document L applies to my start date?
For the council's planning team
- Does permitted development apply at my address?
- Does an Article 4 direction or a listing change that?
What should be on paper
The written price, the product data sheet, any building control notice or approval, and photographs of the framing before it is covered. These documents show the work was done properly.
If something goes wrong
The Citizens Advice consumer helpline, 0808 223 1133, gives advice on disputes with a trader, and a written price with a clear scope makes any such conversation much simpler. Keep your photographs and the data sheet together in one folder.
We survey roof and loft, photograph the structure and give one written price covering window, framing and flashing together. Our guide to roof window flashing kits shows what goes round the frame.
