Window U-Value Requirements UK 2027: Part L, the Future Homes Standard, and What Changes for Replacement Windows
If you’re replacing windows in an existing home, the 2027 rules change almost nothing for you. The Future Homes Standard, published in March 2026 and taking effect from 24 March 2027, is a major overhaul of Part L. Its biggest changes apply to new-build homes, not replacement windows. Your replacement windows still need a U-value of 1.4 W/m²K or better, or a Window Energy Rating of Band B, the same requirement that’s applied since June 2022.
So why does 2027 matter? Because the way in which performance is measured for new builds is fundamentally changing, with industry experts expecting these rules to reach replacement windows eventually. This guide explains what’s changing, what isn’t, and what it means if you’re planning new windows now.
Which U-Value Applies to Your Windows?
Three different U-value figures appear in Part L, and they’re routinely mixed up; each applies to a different situation. Find yours in the table below.
|
Figure |
What it is |
Who it applies to |
|
1.4 W/m²K (or WER Band B) |
The maximum U-value for replacement windows in existing homes. |
Homeowners replacing windows, this is your number, unchanged for 2027. |
|
1.2 W/m²K |
The notional value used to model new-build compliance, from March 2027, is calculated per window using its actual size and configuration, not a standard test size. |
New-build developers and their window suppliers. |
|
1.6 W/m²K |
The backstop, the absolute worst any window can be in a new dwelling, regardless of trade-offs elsewhere in the design. |
New builds only; unchanged from 2021. |
Lower U-values mean less heat loss. Find out more about U-values in our article: Understanding U-Values Before You Buy / Install.
What is Actually Changing?
Until now, U-values could be declared using a standard test window of 1230mm × 1480mm; from March 2027, new-build windows must be calculated at their actual size and configuration.
The per-window calculation shift under the Home Energy Model is significant for new builds: the government is trying to close the gap between theoretical product performance and real-world delivery.
If The Changes Do Affect Replacement Windows, What Are Going to be the Most Affected?
The current timeline is the 24th of March 2027 for standard building work in England. Exactly one year after publication (24th March 2026), people across the industry noticed that this was a notably shorter transition than previous Part L cycles. The 2021 edition of changes gave the industry from December 2021 to June 2022, plus generous transition agreements for work already registered.
These transition arrangements may be provided for projects within the next cycle, as it has been in previous cycles, but this is speculation and hasn’t been confirmed as of writing. The ventilation document was revised at the same time, but it doesn’t change the replacement window requirements. We cover this in our article: Do Replacement Windows Need Trickle Vents? Part F Explained
U-Value or WER Band B: Which Route to Compliance?
WER Band B is another aspect that you will likely come across when carrying out home improvement projects with fenestration, especially for new build projects, and if the changes to Part L do eventually apply to all replacement windows.
Part L gives replacement windows two ways to comply: a maximum U-value of 1.4 W/m²K, or a Window Energy Rating (WER) of Band B or better. Your windows only need to pass one, not both.
U-value and WER measure different things. U-value only measures how heat passes through the window, frame and glass combined. A WER is a broader energy balance, graded like the labels on kitchen appliances from E up to A++. It combines 3 factors: heat loss (U-value), free heat gained from sunlight through the glass, and heat lost due to leakage around the frame.
Because WER credits solar gain, the two routes to compliance don’t always agree. A window can achieve Band B while its U-value sits slightly above 1.4 W/m²K. This is because the sunlight it lets in offsets the heat that it loses. That window would be fully compliant, despite being below the 1.4 limit. Equally, a window can pass on U-value alone without needing a WER certificate at all.