For an existing home in England, the rooflight U-value Part L requires comes as two figures: 2.2 W/m²K for a rooflight, 1.4 W/m²K for a roof window. Ask three suppliers about a skylight U-value requirement and you can easily get three different answers, because the figure on a brochure, the figure on a datasheet and the figure the regulations check are not always the same measurement. This guide sets out the figures, where they come from, why there are two of them, and how to read a quoted number so that it means what you think it means.
What a U-value actually measures
A U-value is the rate at which heat passes through a building element. The unit is W/m²K, short for watts per square metre for every degree of difference between the temperature indoors and out. The lower the number, the less heat escapes. A solid brick wall of the old Victorian sort loses a lot. A modern insulated roof loses very little. Glass sits between the two, and a rooflight is a hole of glass in the best-insulated part of the house, which is why the number gets so much attention.
Think of it as a rate, not a total. The U-value does not tell you how much heat you will lose in total, since that depends on the area of glass, the temperature outside and how long the heating is on. It tells you how leaky each square metre is. A small roof window with a good figure on a landing costs you little. A big lantern with an ordinary figure over a kitchen extension costs you much more, and the same U-value is doing different amounts of damage in each.
The figure belongs to the whole product as installed in a standard test, not to the glass alone. That distinction is the source of most of the confusion, and it is the subject of the fourth chapter.
The figures in Table 4.2
Work on an existing dwelling in England is judged against the 2021 edition of Approved Document L, volume 1, where the figures sit in Table 4.2. When a window, roof window or rooflight is being replaced, the new unit has to meet the limit for its type.
| Product | Maximum U-value | Alternative route |
|---|---|---|
| Roof window | 1.4 W/m²K | Window Energy Rating band B |
| Rooflight | 2.2 W/m²K | None given |
| Roof lantern | 2.2 W/m²K | None given |
The 2.2 figure applies to a roof lantern too, since lanterns are classed as rooflights. A flat rooflight on a kerb, a pyramid and a domed unit are all in the same group.
The 2021 edition is the one that applies today. England gets a 2026 edition from 24 March 2027, which keeps 1.4 and 2.2 for existing dwellings but renumbers the table as Table 3.2. Anything started earlier than that is measured against the 2021 edition, Table 4.2.
Why roof windows and rooflights have different limits
It looks odd that one hole in a roof must reach 1.4 and another may stop at 2.2. The two groups are different products, built and tested in different ways, and the regulations treat them separately.
A roof window lies in the slope of a pitched roof; its frame holds a hinged or pivoting sash, and a handful of big manufacturers make it in catalogue sizes. It is closer to a wall window turned on its side, and it can be rated and compared in the same way. That is why it can use the Window Energy Rating route as an alternative to a plain U-value.
Window Energy Rating bands run from A++ down to E. The rating folds together heat loss, the sun's heat let in through the glass and air leakage round the sash, so a roof window can score well by being tight and sensible as well as by having a low U-value alone. Band B or better meets the limit.
A rooflight, by contrast, covers the units that sit on a kerb: flat rooflights, domes, pyramids and lanterns. These are often large, made to measure and framed in aluminium, and a lantern has a lot of glazed edge and a lot of frame that is hard to insulate. The regulations therefore set a separate, higher ceiling for that group.
It is worth saying plainly what the difference is not. It does not mean a rooflight is allowed to be draughtier or poorer. Both must still be airtight and properly installed. It only means that the numerical ceiling for the heat loss across the product is higher.
The figure on the label and the figure the regulations check are only the same number when they are measured the same way.
How U-values are quoted, and where the trap sits
A glazing supplier can quote at least three numbers, and each is correct in its own context.
The centre-pane figure. This describes the glass alone, away from the edges, and it is always the best-looking number. Left out are the spacer between the panes and the frame around the sealed unit, and both of those leak more heat than the centre of the glass does.
The whole-unit figure. This covers glass, spacer and frame together, and it is the one that corresponds to the limits in Table 4.2. If a datasheet gives a single figure for the product, it should be this one, and it should say so.
The figure for the wrong position. Approved Document L asks for a roof window's U-value in the vertical position and a rooflight's in the horizontal position. Sealed glass loses heat faster lying flat, so a rooflight figure worked out upright looks better than the unit will perform on the roof, and it is easily mistaken for the right one.
A very low centre-pane figure is not evidence of compliance. It is evidence that the middle of the glass is good. If the quote only mentions the glass, ask for the whole-unit figure for the exact product being fitted.
On a kerb-mounted rooflight there is a second place heat escapes: the upstand. The U-value covers the rooflight unit, while the insulation in and around the kerb is part of the roof build-up, so it is worth asking what the kerb is made of and how it is lined. A well-rated rooflight on a bare timber upstand gives away part of what the glass has saved.
Two further points help when comparing quotes. First, a manufacturer's figure is measured on a standard size, and a very large or very small unit will differ a little, because the proportion of frame to glass changes. Second, check that the figure is stated for the right position: horizontal for a rooflight, vertical for a roof window. A datasheet should say which. If it does not, ask.
When the limits apply, and who checks them
The Building Regulations 2010 apply to any new or replacement rooflight, which makes it controlled work. The limits in Table 4.2 are therefore not advice. They are a condition of the work being signed off. The notification is received by the local authority's building control or by a registered building control approver, the term private-sector inspectors have used since April 2024. Once satisfied, a council issues a completion certificate, while an approver's equivalent is the final certificate.
In practice, you see the figure in two places: on the manufacturer's datasheet for the exact product, and on the paperwork at the end of the job. For notifiable work the notification to Building Control comes from us, with the datasheet figure written into it. The wider rules sit in the Building Regulations guide for rooflights, and the difference between replacing like with like and adding a new opening is explained there.
New extensions and conversions sometimes follow a different route, because the building as a whole may be assessed for its energy use and the glazing is one part of that calculation. If your rooflight is part of a larger build, ask the designer or the building control body which route applies. Whichever way it is assessed, a unit that meets Table 4.2 is a sensible starting point.
One more point for owners of older homes. A rooflight that already exists does not have to be upgraded just because the limits changed. The limits apply when it is replaced. So the old single-glazed lantern over a 1930s kitchen extension is the owner's to keep, and the moment it is swapped for a new one, the new one must comply. The same applies when a misted rooflight is swapped for a whole new unit: the new one has to meet the current figure. That is covered on the misted rooflight replacement page.
Wales applies its own document
Building Regulations are devolved. Wales has its own version of Approved Document L, the 2022 edition, and it is the Welsh document that applies to work in Flintshire, Wrexham and the rest of the country. We also work on the Flintshire side of north-east Wales, where the principle matches England's but the document does not: use the Welsh edition instead of the English tables, and ask the building control body to confirm the figure for your job.
Wales has registered approvers as well from April 2024, so an owner in Flintshire or Wrexham can use the county council in Flintshire, the county borough council in Wrexham, or a registered building control approver. The Welsh 2026 edition takes effect on 4 March 2027. If you are comparing a datasheet from an English supplier, check the number against the Welsh document and not against a leaflet written for England.
The Government publishes the English document on gov.uk, at Approved Document L: conservation of fuel and power, and the Welsh Government publishes the Welsh one through gov.wales.
What to ask for on a quote
The figure is only useful if it is attached to the product being supplied. Before agreeing a price, look for these on the quote or the datasheet.
- The exact product name and size, not a range
- A whole-unit U-value, or a Window Energy Rating for a roof window
- Whether the figure is for the right position, horizontal for a rooflight and vertical for a roof window
- Which edition of Approved Document L the unit is being checked against
- Who notifies Building Control, and when
A survey comes first at Dockside, and the fixed written quote follows it. It names the product, the datasheet and the notification, and our workmanship carries a written guarantee that runs for 10 years. The wider range of energy-saving units is on the energy efficient rooflights page. If you want to know whether glass with more panes is worth the extra, the companion guide on double or triple glazed rooflights covers it, and does a rooflight make a room colder in winter deals with how it feels in the room. A WhatsApp message or a free quote request will get things moving.
Awkward U-value situations, answered
Does a lower U-value always mean a better rooflight?
Lower means less heat loss through each square metre, which is the point of the figure. It does not cover light, summer sun, noise or condensation, and a very low number on a very large unit can still lose more heat in total than a modest figure on a small one.
Is Window Energy Rating band B the same as 1.4?
No, they are two routes to the same limit. For a roof window, Table 4.2 allows two ways in: a whole-unit U-value of 1.4 or lower, or a Window Energy Rating in band B or better. A rooflight has no rating route, so it must meet the U-value.
Do I need to upgrade a rooflight I am not replacing?
No. The limits apply to the new unit when one is fitted. A rooflight that stays as it is does not need to be brought up to the current figure.
Will the 2026 edition change the figures?
For existing dwellings in England the figures stay at 1.4 and 2.2. On 24 March 2027 the 2026 edition arrives and the table takes the number 3.2.