What U-Value Must a New Roof Window Meet?

Part L of the Building Regulations deals with saving fuel and power, and Approved Document L, Volume 1, is its guidance for homes. The 2021 edition, with 2023 amendments, has applied in England since 15 June 2022. Table 4.2 is the one for new or replacement parts of an existing dwelling.

What is a U-value, and what is Band B?

Think of a U-value as the speed at which heat leaks through a wall, roof or window, written W/(m²·K). A lower number means less heat lost. The Window Energy Rating, or WER, grades windows into lettered bands, and Table 4.2 accepts Band B as the floor.

The window row asks for 1.4, or Band B at the least. Its note 8 says the row covers roof windows as well as curtain walling. So a roof window going into an existing house, as a replacement or an addition, should meet that standard. Get the U-value or WER band from the supplier's product data before you order.

Keep that datasheet with your records. Building control may ask for it, and it settles any later doubt about what was fitted, which is a small step that saves an argument.

Table 4.2 carries a separate row for rooflights, and the number there is 2.2. Swapping the two is the commonest slip in articles on this subject, so it is worth being clear about which is which.

Is a roof window the same thing as a rooflight?

Not in the Building Regulations. A roof window is a window made for a pitched roof, usually with a sash that opens, and it takes the window figure of 1.4 or WER Band B. A rooflight is another product altogether, a glazed element set in or on a roof, such as a fixed light in a flat roof, and its figure is 2.2. Our answer on skylights and roof windows sets out the difference.

The mix-up has consequences. Quote 2.2 against a roof window and you are working to a figure that is too relaxed, so the window may fail the check. In the other direction, a rooflight does not need to reach 1.4. If someone hands you a figure for a product, ask which row of the table it answers to and which edition it comes from.

The table also gives a limit of 0.35 W/(m²·K) for an upstand built on site, which matters where a rooflight sits on a kerb.

Table 4.2 is about new parts. Table 4.3 covers parts that already exist, such as a roof being renovated or kept in a loft conversion, and its numbers differ. The roof threshold there is 0.35, and the improved figure is 0.16 W/(m²·K).

Does the roof around the window need upgrading too?

It can, if the work counts as renovating the roof. A note to Table 4.3 treats the roof parts of dormer windows as roof. Paragraph 4.13 then gives some room. If upgrading is not workable, or would need over 15 years to pay for itself, you aim for the best result that is feasible and still repays within 15 years. As a general ceiling, an upgraded element should not be worse than 0.7 W/(m²·K).

For a renovation or a loft conversion, the figures to work from are the Table 4.3 pair of 0.35 and 0.16. The numbers in Table 4.2 for new parts are a separate matter. Our page on Building Regulations for roof windows covers when approval applies.

A new edition of Approved Document L is coming. The 2026 edition starts on 24 March 2027 in England, leaving out work tied to higher-risk buildings. The tables change number, 3.2 for new elements and 3.3 for existing ones, and the 15-year payback rule moves to paragraph 3.14.

Which edition applies to my window?

Before 24 March 2027, cite the 2021 edition and its Tables 4.2 and 4.3. After that date, building work follows the 2026 edition. Building control can say which applies to a job starting close to the changeover. Whichever edition is in force, the practical point stays the same: specify the whole window, not just the glass, and keep the datasheet.

Look for the U-value or WER band printed on the label, and hand the datasheet over if building control requests it. A roof window in a loft conversion, a swap or a new opening should meet the figure, and the building control body decides on compliance.

We look at the existing opening and the roof around it during the survey, explain what the work involves and give one written price. For cost, see Velux window installation cost, or start from roof windows and skylights.