Quick answer: a roof window sits in the plane of the roof and is treated as a window — limiting U-value 1.4 W/m²K, assessed in the vertical position. A rooflight sits out of plane on a kerb or upstand — limiting U-value 2.2 W/m²K, assessed horizontally. Both count towards the 25% opening allowance for an extension. Use the manufacturer's whole-unit value in the position the unit will actually be installed, never the centre-pane figure.
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The distinction is position, not product
| Roof window | Rooflight | |
|---|---|---|
| Where it sits | In the plane of the roof, between the rafters | Out of plane, on a kerb or upstand |
| Typical use | Pitched roofs, loft conversions | Flat roofs, lanterns, walk-on units |
| Limiting U-value | 1.4 | 2.2 |
| Assessed in the | vertical position | horizontal position |
| Counts to the 25% allowance | Yes | Yes |
The same unit tested flat gives a worse U-value than tested upright — convection in the cavity works against you when the glass is horizontal. That is the whole reason for two numbers: the limit follows the way the unit will be used, so the horizontal limit is more generous.
Why it matters more than it looks
Two ways round, and both cost money. Label a flat-roof lantern as a roof window and you have set yourself a 1.4 target it may not meet, and an extension that would have complied gets sent back. Label a pitched roof window as a rooflight and you have claimed a 2.2 limit it is not entitled to — Building Control may accept it, and the calculation is wrong either way.
In an area-weighted calculation it swings the answer as well, because the notional extension credits rooflights at 2.2. For a 1.2 × 1.5 m unit (1.80 m²), the notional allows 3.96 W/K against 2.52 W/K — a difference of 1.44 W/K in your favour, per unit, purely from being classified correctly. Specify that rooflight at 1.2 rather than 2.2 and you bank 1.80 W/K of headroom for the rest of the extension.
Getting the right number out of a supplier
- Ask for the whole-unit U-value — frame, kerb and glass together, to BS EN ISO 12567 or the relevant product standard. A centre-pane figure flatters the unit by a wide margin and is not what Part L asks for.
- Ask which position it was assessed in, and match it to how the unit is being installed. Many manufacturers publish both.
- Check whether the kerb is included. An uninsulated upstand can undo a good unit, and some published figures assume a kerb specification you are not buying.
- Keep the evidence. Building Control will want the declaration that goes with the number, not just the number.
They still count towards the allowance
Whatever they are called, their area goes into the same total as the windows and doors, against 25% of the extension's floor area plus any openings the extension covers over. A 3 m lantern over a small kitchen extension can eat the whole allowance before a single window is counted, which is usually the moment an area-weighted calculation becomes necessary.
Frequently asked questions
What is the difference between a rooflight and a roof window?
Position. A roof window sits in the same plane as the roof, between the rafters, flush with the covering. A rooflight sits out of plane, on a kerb or upstand above the roof line — the flat-roof lanterns and "walk-on" units. Approved Document L gives them different limiting U-values because they are tested in different positions.
What U-value does a rooflight need?
2.2 W/m²K for a rooflight on an upstand, assessed in the horizontal position. A roof window in the plane of the roof is treated as a window: 1.4 W/m²K, assessed vertically. The same physical unit produces a worse number lying flat than standing up, which is why the two limits differ.
Do rooflights count towards the 25% glazing allowance?
Yes. The allowance covers the total area of windows, roof windows, rooflights and doors together, against 25% of the extension floor area. A lantern over a small kitchen extension can use most of the allowance on its own.
Why does my supplier quote two different U-values for the same unit?
Because of the test position. Manufacturers commonly publish both a vertical and a horizontal figure, and sometimes a centre-pane value as well. For Part L you need the whole-unit value in the position the unit will actually be installed — and the centre-pane figure is never the right one.
Is a flat rooflight worse than a roof window?
Thermally, in the same product, yes — a horizontal unit loses more heat than the same unit at pitch, which is why it is assessed that way and given a more generous limit. In an area-weighted calculation the notional extension also credits the rooflight at 2.2, so a rooflight specified well below that earns headroom for the rest of the design.
Next: every limiting U-value for an extension · the area-weighted calculation · work out a roof build-up free.
Basis: Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England), Table 4.2 and its notes on the assessed position of roof windows and rooflights.