Part L is the Building Reg that governs energy performance in buildings. Here's what it means for your windows and doors.
Approved Document L was updated in 2022 with tightened targets ahead of the 2025 Future Homes Standard. For window replacement in existing dwellings, the current requirement is a whole-window U-value of 1.4 W/m²K or better, or a Window Energy Rating (WER) of C or above. Doors have separate targets (1.4 W/m²K for external doors).
U-value measures heat loss per m² per °C. Lower is better. WER (Window Energy Rating) bundles U-value with solar gain and air leakage into a single A++ to G scale, similar to appliance ratings. Modern double glazing achieves U-value 1.2 (WER A) easily; older units sit at 2.0 and above (WER D or E).
Every window quote should state the U-value or WER for each opening. If it doesn't, ask. A reputable installer will provide manufacturer certification (from the BFRC or Certass) confirming the achieved rating for the specific product on your order.
For brand-new dwellings, targets are tighter — typically U-value 1.2 or better on windows and 1.0 on doors, moving to 0.8/0.8 with the 2025 Future Homes Standard. This is why triple glazing is becoming standard on new builds — double glazing struggles to meet the tightest targets.
Listed buildings, some conservation areas, and specific heritage properties are exempt from Part L on windows where compliance would harm the character of the building. This is why timber sashes with 14mm slim units (U-value around 2.0) can still be legally installed on listed buildings.
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