The short answer
Approved Document L expects a roof window that you add or swap in an existing house to reach a U-value of 1.4, or else a rating of at least Band B under the Window Energy Rating scheme. The 2.2 figure belongs to rooflights, a separate row in the same table. The 2026 edition of the document applies from 24 March 2027.
The figure in Table 4.2
Part L of the Building Regulations is about saving fuel and power, and Volume 1 of its approved document covers dwellings. Table 4.2 sets the targets for parts of a house that are new or being replaced. For windows it gives 1.4 as the U-value, with Window Energy Rating Band B as the minimum alternative, and a note on that row says it covers roof windows and curtain walling.
So a roof window follows the window figure. The U-value is written W/m2K, the watts lost through each square metre of glass and frame for every degree of temperature gap, and a lower number means less heat escapes. A rating band offers a second way to comply, because the scheme also credits a window for free heat from the sun.
The 2021 edition of the document took effect on 15 June 2022. Its successor, the 2026 edition, takes effect on 24 March 2027 for work that is not connected with higher-risk buildings, and renumbers the same tables as 3.2 and 3.3. Until that date, the 2021 tables are the working reference.
Each product carries its figure on its data sheet, which we check before ordering.
Why 2.2 is a different product's number
The same table has a separate row for a rooflight, and that row's figure is 2.2. A rooflight is not the same as a roof window under the document's table, so applying 2.2 to a roof window would let in a product that fails the 1.4 standard. It is a common slip, and a data sheet that quotes a figure with no note of which product it describes is worth querying.
Other figures sit nearby and cause more confusion. The roof itself has a new-element target of 0.15, and for existing roofs Table 4.3 sets a 0.35 threshold with an improved 0.16, which applies to renovation and loft conversions. Those numbers describe insulation in the roof, not the glass in a window set into it.
If you are building a loft room, the roof insulation and the window each have their own target, and the survey notes both. Our answer on rooflights and roof windows explains the difference in the words that the trade uses, and why the table treats them separately.
When the rule applies to your job
In the 2010 Regulations a thermal element is a wall, floor or roof only. Roof windows and roof-lights sit outside that definition, as do doors and ordinary windows. So a roof window is judged as a window in its own right. Its own row in Table 4.2 applies when one is installed or replaced, and the renovation rules for layers of roof construction do not stand in for it.
In practice, fitting a new roof window where there was none, or swapping an old unit for a new one, is the moment to meet the standard. A like-for-like swap is still a replacement window, and the replacement should reach the figure. Our guide on replacing a roof window like for like goes through the steps.
Your building control body decides how the work is handled for your address. We ask it what it needs for the job, and tell you the answer before the work begins.
Keep the data sheet and the invoice together. They show which product went in and what its figure was, and a buyer's conveyancer may ask for them.
What a better U-value does in daily use
A lower U-value keeps the inside pane warmer, and a warmer pane is less likely to mist with condensation on a cold morning. It also means less heat leaving a loft room, which matters because warm air gathers at the top of the house and the glass is the thinnest part of the roof. Over a winter, the difference shows up in how the room feels near the window as much as in a bill.
Frame and spacer details count as much as the glass itself. A window with a good figure on paper but a cold metal frame can still collect moisture along the edge, a point our answer on skylights and thermal breaks covers. Triple glazing and low-emissivity coatings (a thin metallic layer on the glass that reflects heat back into the room) both push the number down, at a higher purchase price.
Energy-saving materials can be zero-rated for VAT until 31 March 2027, then move to 5%. Roof windows are not on that list, and HMRC treats double glazing as standard-rated, so expect VAT at 20%. Our roof windows and skylights page covers what we fit.
