The short answer
Fitting a roof window to an existing English house means reaching 1.4 W/m²K for heat loss, or else Band B on the industry's energy rating scale. The figure comes from Table 4.2 of Approved Document L, 2021 edition. Rooflights over flat roofs are judged on their own 2.2 W/m²K limit, so confirm which product type your quote describes.
The 1.4 figure and where it comes from
Approved Document L, the government guidance on conservation of fuel and power, took effect in England on 15 June 2022. Table 4.2 covers new and replacement elements fitted to an existing dwelling. For windows, its note says the entry includes roof windows, and the limit is written as "1.4 or Window Energy Rating Band B minimum", so either measure will do.
Heat loss is what a U-value tells you: how many watts pass through each square metre for every degree of difference between inside and out. Lower is better. The energy rating scale works differently, grading a window in bands and giving it some credit for warmth the glass lets in from the sun. Either route satisfies the rule, which is why a data sheet may print one, the other or both.
Table 4.2 also sets the roof itself at 0.15 W/m²K. That is worth knowing when a window goes into a pitched slope, because glazing will always leak more heat than the insulated roof around it. A well-made unit narrows that gap without closing it.
Timber frames and the 2023 change
Timber windows were given a slightly easier limit for a time. The guidance allowed 1.6 W/m²K or Window Energy Rating Band C until 14 June 2023, and the full standard has applied to every frame material from 15 June 2023. Many roof windows have timber frames, often with a polyurethane or painted finish, so you may see older data sheets quoting the looser figure.
If a window listing or an old quote shows 1.6, it predates the change. For work now, ask for a data sheet that shows the whole-window figure, not only the centre-pane value for the glass. The centre of the pane always performs better than the frame edge, and the rule is about the complete unit.
Roof window or rooflight: why the figure differs
Part L draws a line between the two products. A roof window sits in a pitched slope and opens or tilts like a window, so it follows the 1.4 limit. A rooflight, the glazed unit set on a kerb or upstand above a flat roof, follows a looser 2.2 W/m²K. The area used is the outer developed surface, meaning the whole outside face including any sloping glass or dome, not just the opening beneath.
Two further details trip people up. A rooflight supplied as a single assembly with its own kerb is judged as one unit at 2.2. A kerb built on site is judged separately, and the upstand is limited to 0.35 W/m²K. If your project is a flat roof, our guide to rooflights for flat roofs compares fixed, opening and walk-on types. For pitched slopes, the 1.4 limit applies and the reasoning below is the same.
| Element | Limit in Table 4.2 |
|---|---|
| Roof window | 1.4, or Band B on the rating scale |
| Rooflight | 2.2 W/m²K |
| Site-built upstand | 0.35 W/m²K |
| Roof | 0.15 W/m²K |
What replacing an old window changes
Swapping a worn roof window counts as a replacement element. Paragraph 4.8 of the guidance asks that the new element is no worse than the old and also meets the limiting standards in Table 4.2. So copying an old, poorly performing unit is not an option: the replacement has to reach 1.4 or Band B.
That is helpful, because most of the gain from a new window comes from the glazing and the seals. Low-emissivity coatings, gas fill in the cavity and an insulated frame all feed into the final figure. If you are weighing like-for-like against an upgrade, our guide to roof windows and Building Regulations sets the rest of the rules around the thermal one, including structure and sign-off.
A cheap unit that falls short of the figure can be refused at inspection, which costs more than the saving. Our office deals with the Building Regulations paperwork for notifiable jobs, and we explain which consent applies. The notification goes either to your council's building control or to a registered building control approver.
Reading a data sheet and checking the label
Ask for the product reference and the manufacturer's data sheet before you agree a quote. Look for three things: the U-value of the whole window, whether it is quoted to the standard test method, and the glazing build-up. A low figure on its own may describe only the centre of the glass. The line you want states that it applies to the complete window.
Other points worth checking on the sheet:
- The frame material and any thermal break in the frame.
- Whether the figure includes the standard flashing kit or the product on its own.
- Whether it is a rooflight or a roof window, because the limit differs.
- The date of the sheet, since specifications are revised.
On price, MyBuilder's 2026 guide to roof window costs gives £350 to £600 for fitting on straightforward jobs, £305 for a standard centre-pivot unit, and about £500 for scaffold. A higher-specification glazing build-up moves the product price rather than the labour. Following the survey you get a written quote covering the window, flashing kit, scaffold and making good.
Once you have the figure, a cross-check against the space is worth doing. Our answer on what size roof window you need covers the dimension side, and the insurance answer explains what cover usually expects from a sound installation. The wider range is on our roof windows and skylights page, and the finished fitting carries our 10-year workmanship guarantee in writing.