Part L is the specific regulation that sets the minimum energy performance for replacement windows and doors. Here is what it actually requires.
Part L of the Building Regulations covers the conservation of fuel and power across a building, and for existing dwellings it sets a specific minimum standard for replacement windows and external doors. It is the regulation referenced whenever a supplier says a window is 'Part L compliant', and it is worth understanding what that phrase actually guarantees.
This guide focuses specifically on Part L as it applies to replacement glazing in existing homes, which is the situation most homeowners in Cumbria are dealing with.
For a replacement window or door in an existing dwelling, Part L requires a whole-window (or whole-door) U-value of 1.4 W/m²K or better, or, as an alternative route to compliance, a Window Energy Rating of band B or better. Either route is acceptable; a Window Energy Rating is a simpler, single-figure measure that also accounts for solar gain and air leakage, not just heat loss.
| Route to compliance | Standard required |
|---|---|
| Whole-window U-value | 1.4 W/m²K or better |
| Whole-door U-value | 1.4 W/m²K or better |
| Window Energy Rating | Band B or better |
It is worth checking that any U-value quoted is a whole-window figure, which accounts for the frame and spacer bar as well as the glass, rather than a centre-pane figure, which only measures the glass itself and will always look more favourable. Part L compliance is assessed against the whole-window figure, so this is not a minor technicality.
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