A roof window looks like a simple product that arrives in a box, gets fixed between two rafters and lets light into the room. In regulatory terms it is three things at once. It is a part of the building's outer shell, so the energy rules apply. In a loft room it may be the way out in an emergency, so the fire safety rules apply. And it needs an opening through the rafters, so the structure has to be thought about. Each question has a different document behind it, and each is answered by a different part of the Building Regulations.
This guide takes the three in turn. It sets out the energy figures in Approved Document L, the escape window sizes in Approved Document B, and the practical points about framing an opening in a loft. It also explains how the planning limits for a roof window differ from the building rules, and what to ask before ordering. Treat it as general guidance rather than legal advice. The building control body for your address gives the ruling that counts, and we put the right questions to it before an order goes in.
When is a roof window a Building Regulations job?
The answer depends on what you are doing, and on which council covers your address. Replacing an existing roof window with the same size in the same opening is a different job from cutting a brand new opening, and councils do not always word it the same way. The sensible habit is to ask before the order, and not to assume the answer from a neighbour's experience.
What does Crawley say?
On Crawley Building Control's list of domestic work needing approval, a new roof window appears as an altered opening, so a Crawley address needs its sign-off for one. That is the borough's own wording. For other councils around us, the building control body named on the council's website is the one to ask, and Horsham, Mid Sussex and the Surrey councils each have their own arrangements.
How does the Planning Portal test treat roof windows?
The Planning Portal's re-roofing guidance counts roof windows and rooflights as part of the total building envelope, alongside the walls, floor, roof and doors. It sets two thresholds, 25 per cent of the envelope and 50 per cent of the roof, and work below both is normally exempt from an application. A single roof window is a small part of either figure. That does not decide the question on its own, because a new opening changes the roof structure and not only its surface, and the council can take a different view. Our page on replacing a Velux window covers the like-for-like case.
Part L and roof window U-values
Approved Document L covers the saving of fuel and power. A U-value records the heat lost through one square metre of a material when the air on either side differs by one degree, so a lower number means less waste. Roof windows lose more heat than the insulated roof around them, which is why the regulations set a limit on them.
Which figures apply?
For elements newly fitted or swapped in a house that already exists, Table 4.2 of the 2021 edition (with 2023 amendments) gives a window figure of 1.4 W/(m²·K), with Band B the lowest acceptable Window Energy Rating, and a rooflight figure of 2.2. The same table gives a roof figure of 0.15, and says an upstand built on site should not exceed 0.35. Note 8 to the table puts roof windows in the window row, so a roof window works to 1.4 and a rooflight to 2.2; we check the product's own test data with your building control body.
Which edition applies now?
The 2021 edition took effect on 15 June 2022 and remains in use for now. The 2026 edition takes effect on 24 March 2027 for most building work, with the equivalent figures moved to Table 3.2. The numbers for windows and rooflights are unchanged in that table. Ask your supplier for the U-value of the exact product, because the figure is printed in its data sheet, and have it quoted in writing so building control can see it. A good product with a poor installation will still leak heat, so the fit and the insulation around the frame matter as much as the glass.
Means of escape from a loft room
When a roof window is part of a loft conversion, it can also form part of the escape plan if there is a fire. This is covered by Approved Document B, the fire safety volume for dwellings. Our page on the fire escape roof window for a loft goes into more detail, and the main points are these.
Why does 4.5 metres matter?
Paragraph 2.5 deals with a storey more than 4.5 m above the ground. For such a storey there are two routes: a protected stairway, built with fire resisting construction of at least 30 minutes, or an alternative escape route, where a top storey is separated from the lower floors by fire resisting construction and leads to its own final exit. Paragraph 2.21 applies this to loft conversions and asks that the full extent of the escape route be addressed, with fire resisting doors and partitions provided and existing doors upgraded where needed. On a two-storey house, a converted loft normally calls for a protected stair, although Approved Document B leaves room for alternatives.
What size must an escape window be?
Paragraph 2.10 sets the minimum clear openable area at 0.33 m², with a height of at least 450 mm and a width of at least 450 mm. The bottom of the openable area must be no more than 1100 mm above the floor. Paragraph 2.23 gives one alternative route that relies on sprinklers in the open-plan areas and a fire resisting partition and door, leaving loft occupants with access to a first-floor escape window. Check the actual opening size of the model you pick, not the nominal frame size.
What happens to the structure when rafters are cut
A roof window needs an opening, and the opening interrupts the rafters. Each rafter carries a share of the roof's load down to the wall plate, so the load that used to go straight down has to be taken round the hole. That is why a new opening is a structural matter and not just a carpentry one.
How is an opening framed?
The usual way is to fix timber across the gap above and below the window, called trimmers, and to double up the rafters at each side so the load of the cut rafter travels round the frame. The sizes of those timbers depend on the span, the pitch, the covering and the width of the opening. A heavier covering, such as concrete tiles, puts more load on the trimmers than lighter ones. For a wide opening, or one at the middle of a long rafter span, ask whether a structural engineer's calculation is needed, and ask building control whether it wants to see it.
Is any of the framing on show?
In a loft conversion the framing sits inside the room's lining, so the finish hides it. That is the reason inspections can matter: building control may want to see the framing before the plasterboard goes on. Tell the roofer your timetable and who is doing the lining, so the inspection is not missed. Flashings, the sheet metal that seals the window to the covering, are then fitted to the tile or slate profile. Our guide to roof windows and skylights explains the full job from the survey onward.
Planning limits that sit beside the building rules
Planning rules are separate from the building rules. For a house, roof windows are generally dealt with under Class C of Part 1 of Schedule 2 to the General Permitted Development Order 2015, the catch-all for roof alterations to a single home. Class C does not allow a window that protrudes more than 0.15 metres beyond the plane of the original roof slope, and it does not allow work that makes the highest part of the roof higher than the original.
What about a roof window on a side slope?
Condition C.2 treats a window in a side-facing slope more strictly. It must be obscure-glazed. Separately, it cannot open unless the opening parts sit higher than 1.7 metres from the floor of its room. The first rule never relaxes, and only the second does.
Do conservation areas and listed buildings change it?
Class C carries no exclusion for conservation areas or National Landscapes. A roof window inside the limits can therefore stay permitted development in those places, unless an Article 4 direction or a planning condition has taken the right away. A dormer is different, since permitted development does not cover dormers on article 2(3) land. A listed house keeps its Class C rights but needs listed building consent where the work alters its character. Flats sit outside the order, since the legal term dwellinghouse leaves out any building that contains them. For period homes, see our page on conservation rooflights.
What to settle before the window is ordered
Roof windows are made to size, so a wrong decision at the order stage becomes an expensive one. The checks cost nothing and they take place before anyone cuts a tile.
Which questions should you ask?
Ask who handles the application to building control, and whether it is needed for your address. Ask for the U-value of the exact product and the opening size of its openable area. Ask whether the rafters will be cut and whether an engineer's calculation is being prepared. Ask who inspects the framing, and when. Ask if the quote is one total price covering the window, flashings, scaffold and VAT, which the government gives as 20% at the standard rate. Ask also for the lead time on the product, since a made-to-size window can take a while to arrive and the scaffold date should follow it.
What helps us most at the start?
Pictures of the room and the roof slope, the spot you have in mind and roughly when the house was built give us a head start. Say whether the room is a loft conversion or an existing bedroom, whether the house is listed or has conservation area status, and whether the slope faces the side boundary. During the survey we measure between the rafters and look at the covering, and then we put the escape, energy and structural questions to your building control body first. The single written price follows what it confirms.
Part of our Velux Roof Windows & Skylights in Crawley
Roof window rule questions
Does a roof window have to meet a U-value of 1.4?
Yes, if it is a new or replacement unit. Table 4.2 of Approved Document L gives 1.4 for windows, or Window Energy Rating Band B at minimum, and its notes count roof windows as windows; 2.2 is for rooflights. Ask for the data sheet and let building control confirm.
How big must an escape window in a loft be?
Approved Document B paragraph 2.10 asks for an openable area of at least 0.33 m², with a height and width of at least 450 mm each. The bottom of that openable area must sit no higher than 1100 mm above the floor. Check the figures against the actual opening of your chosen model.
Is a roof window permitted development on a side slope?
Under Class C, a window in a side-elevation slope has to have obscured glass without exception. It must also stay shut unless the opening parts sit over 1.7 metres above the floor. Nothing may protrude more than 0.15 metres from the roof plane. Your council's planning team can confirm whether any direction removes these rights.
