Search for building regulations for replacement windows and many of the top results still quote the old rules. Some say a U-value of 1.6 is fine. Some still mention Band C. Those figures were replaced in June 2022.
Here's what actually applies to a house in England today, in plain English.
1. Replacement windows have to meet Part L
Replacement windows and doors count as "controlled fittings" under the Building Regulations. Under Approved Document L (2021 edition), a new window in an existing home should reach a U-value of 1.4 W/m²K or better, or a Window Energy Rating of Band B or better. The new window also can't be worse than the one it replaces.
Timber windows had a temporary allowance of 1.6 / Band C. That ended on 14 June 2023. The same 1.4 / Band B figure now applies to timber too.
A 2026 edition of Approved Document L has also been published. It lists the same 1.4 / Band B figure for windows.
2. What older guides get wrong
- "1.6 W/m²K is the limit": that was the old rule. It's 1.4 now.
- "Band C is fine": not for replacement windows. It's Band B or better.
- "Trickle vents are optional": not always. See point 3.
- "You need a FENSA certificate": you need a compliance certificate from a competent person scheme. FENSA is one scheme. CERTASS is another. Both are valid.
3. Don't make ventilation worse
Approved Document F (2021) covers ventilation. When windows are replaced, the home shouldn't end up with less ventilation than before. If your old windows had trickle vents, the new ones should have them too. This matters for condensation. Sealing a house up tight without ventilation is one of the main causes of wet windows and black mould.
4. Keep your escape windows
Some upstairs rooms need a window big enough to escape through in a fire. A replacement window shouldn't make that opening smaller. A good surveyor will check this before ordering.
5. Older or listed homes
Where a building's character means a window can't meet 1.4, Approved Document L allows other routes, such as a centre-pane U-value of 1.2 or adding secondary glazing. Listed buildings and some conservation areas also have planning rules. Check with Warrington Borough Council before you change anything.
6. Your compliance certificate
If your installer is registered with a competent person scheme, they register the job with the scheme. The scheme then tells the council and you get a certificate. Keep it safe. Solicitors ask for it when you sell.
If the installer isn't registered, you, or they, have to apply to building control separately. That takes longer and costs a fee.
Clear Choice is CERTASS registered, so we self-certify the windows and doors we fit and register them for you.
Sources
Questions
What U-value do replacement windows need in 2026?
In England, a replacement window should reach a U-value of 1.4 W/m²K or better, or a Window Energy Rating of Band B or better. It also can't be worse than the window it replaces.
Is a U-value of 1.6 still allowed?
Not for most replacement windows. 1.6 was the old figure. The temporary 1.6 allowance for timber windows ended on 14 June 2023.
Do new windows need trickle vents?
If your old windows had trickle vents, the new ones should too. Replacing windows shouldn't leave the home with less ventilation than before.
Do I need a FENSA certificate?
You need a compliance certificate from a competent person scheme, or building control approval. FENSA and CERTASS are both competent person schemes.
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