Part L of the Building Regulations covers the conservation of fuel and power, and it sets the energy standard your new windows have to reach. If you are an ordinary homeowner swapping tired windows for new ones, the rule is simpler than the jargon suggests. Each replacement window must achieve a whole-window U-value of 1.4 W/m²K or lower, or carry a Window Energy Rating (WER) of band B or better. Meet either route, get a FENSA certificate, and your job is compliant.
The rule for replacement windows
For a replacement window in an existing dwelling, Approved Document L gives you two ways to comply. The first is the U-value route: the whole-window U-value must be 1.4 W/m²K or lower. The second is the rating route: the window must carry a Window Energy Rating of band B or better. You only have to satisfy one of them, not both. There is also a backstop principle that the new window must be no worse than the one it replaces, so you cannot downgrade performance even if the headline figure is met. This is the standard that applies to nearly every homeowner reading this, because it covers replacement and retrofit rather than new construction.
I sign off Part L compliance on every job I fit, day in, day out. The honest truth is that any decent A-rated unit from a reputable supplier clears 1.4 W/m²K without you lifting a finger. The number scares people, but in 2026 you would have to go out of your way to buy a window that fails it.
Tom Bradley, FENSA installer
What changed in June 2023
The threshold used to be 1.6 W/m²K. From 15 June 2023, Approved Document L (Conservation of fuel and power), Volume 1: Dwellings, tightened it to 1.4 W/m²K for replacement work in England. The WER band B alternative came in alongside it. If you replaced windows before that date, your old certificate reflected the looser 1.6 figure, which is one reason to check any windows fitted in the last few years still meet today's rules before you sell. Understanding how the headline number is measured is worth a few minutes, so it is covered in detail in our guide to U-values.
How U-values stack up
A U-value measures how much heat passes through the window, in W/m²K, so lower is better. The table below shows where each glazing type sits. Anything at or below 1.4 W/m²K satisfies the U-value route, and you can see that modern A-rated double glazing already does, with triple glazing going further still.
| Glazing type | Whole-window U-value | Notes |
|---|---|---|
| Single glazing | ~4.8-5.8 W/m²K | Old, uninsulated. Loses the most heat. |
| Old double glazing (pre-2002) | ~2.8-3.2 W/m²K | No low-E coating or gas fill. |
| Modern A-rated double glazing | 1.0-1.4 W/m²K | Low-E glass, argon fill, warm-edge spacer. Meets Part L. |
| Triple glazing | 0.6-1.0 W/m²K | Best performance, around 0.8 typical. Heavier and pricier. |
Source: site U-value dataset (lib/data.ts). Lower values lose less heat. The Part L threshold for replacement windows is 1.4 W/m²K or WER band B.
The practical takeaway is that you do not need to overspend on triple glazing to comply. A standard A-rated double-glazed unit, built with low-E glass, an argon fill and a warm-edge spacer, lands between 1.0 and 1.4 W/m²K and meets the rule. If you want to see how the glass build-up changes both performance and price, our glass specification cost breakdown sets the options side by side.
How compliance gets certified
You do not have to deal with the council yourself. A FENSA-registered installer (or one in another competent-person scheme such as Certass) self-certifies that the replacement windows meet Building Regulations, including Part L, and issues you a certificate. That avoids a separate Building Control application, which is slower and adds cost. The certificate is also the document a buyer's solicitor will ask for when you sell, so it is worth keeping safe. If your installer cannot tell you which scheme they belong to, treat that as a warning sign and look elsewhere.
People assume Part L means a planning headache. It does not. The whole point of FENSA is that I certify the work and you never speak to Building Control. Just make sure the certificate is part of the price you agree, in writing, before any glass goes in.
Tom Bradley, FENSA installer
New build context: the Future Homes Standard
You may have seen a tighter figure of 1.2 W/m²K in the press. That belongs to the Future Homes and Buildings Standard (FHBS), which applies to new dwellings only. Under the FHBS, windows and doors must reach 1.2 W/m²K or lower, and the notional wall U-value tightens from 0.18 W/m²K under Part L 2021 to 0.15, although the backstop limiting value stays at 0.26. The standard is expected to roll out across 2025 and 2026, with the aim of building homes that are zero-carbon ready. None of this changes the rule for replacements. If you are swapping windows in an existing home, your floor remains 1.4 W/m²K or WER band B. Think of the 1.2 W/m²K figure as a glimpse of best-in-class performance, not a target you are obliged to hit.
What this means for your project
In short, buy A-rated double glazing from a reputable installer, confirm the quote states the windows meet Part L, and make sure a FENSA or Certass certificate is included. That combination covers the regulations without any extra work on your part, beyond the trickle vents the same rules typically require for background ventilation. Once you know the spec you want, the uPVC window calculator gives you a fair price to check quotes against, so you can tell a sensible figure from an inflated one before you commit.
Frequently asked questions
A replacement window in an existing home must achieve a whole-window U-value of 1.4 W/m²K or lower, or carry a Window Energy Rating (WER) of band B or better. Either route satisfies Approved Document L. The replacement also has to be no worse than the window it takes out.
Since 15 June 2023. Before then the limit for replacement windows was 1.6 W/m²K. Approved Document L (Conservation of fuel and power), Volume 1: Dwellings, tightened it to 1.4 W/m²K for work in England.
No. The 1.2 W/m²K figure belongs to the Future Homes and Buildings Standard, which governs new-build dwellings only. For replacing windows in an existing home, the floor stays at 1.4 W/m²K or WER band B. Treat 1.2 W/m²K as best-in-class context, not a requirement for your job.
A FENSA-registered installer (or one in another competent-person scheme such as Certass) self-certifies the work and issues you a certificate. That avoids a separate Building Control application to the council, which is slower and costs more.
Double glazing is fine. Modern A-rated double glazing sits at roughly 1.0 to 1.4 W/m²K, so it meets the Part L threshold comfortably. Triple glazing (around 0.6 to 1.0 W/m²K) exceeds it but is not required for a replacement.
If the existing window already performs better than 1.4 W/m²K, the new one must at least match that performance rather than fall back to the 1.4 limit. In practice this only bites if you are replacing fairly recent units, because most older windows are well above 1.4.
Non-compliant replacement work is unauthorised. When you sell, the buyer's solicitor will ask for the FENSA or Building Regulations certificate, and a missing or non-compliant one can stall the sale or mean paying for retrospective approval or indemnity insurance.