Does this change what my replacement windows have to meet?
Almost certainly not, and it is worth saying so before anything else, because the headlines do not make the distinction. The Future Homes Standard is a new-build standard. Building Circular 01/2026, published on 24 March 2026, sets out changes aimed at new dwellings and new building work. The single new functional requirement it creates, L3, is for the provision of a system of on-site renewable electricity generation for new dwellings and buildings containing dwellings.
Read that scope again, because it is the whole answer. New dwellings, and buildings containing dwellings. If your house is already standing and you are taking out tired units and putting better ones in, this circular is not the rule that governs your job. The rule that governs your job is Approved Document L, and our guide to Part L for replacement windows sets out the site's sourced position on it: a maximum whole-window U-value of 1.4 W/m²K, or a Window Energy Rating of band B or better, in force since 15 June 2023, with the replacement also having to be no worse than the window it takes out.
What the circular actually says
The instrument is The Building Regulations etc. (Amendment) (England) Regulations 2026. It comes into force on 24 March 2027, except that regulations 3, 4, 6, 7, 8 and 9 come into force for higher-risk building work, and for work to existing higher-risk buildings, on 24 September 2027, subject to transitional provisions. That is the point at which the Future Homes Standard stopped being a consultation exercise and became a dated piece of law with commencement provisions attached to it.
Note the name of the instrument. It is an England measure, and building regulations are devolved, so Scotland, Wales and Northern Ireland run their own standards on their own timetables. That is the same split we flagged when the EPC C deadline for landlords was confirmed for England and Wales, and it catches people out every time.
L3 is about electricity, not glazing
The functional requirement being created is for on-site renewable electricity generation. In practical terms that is the solar obligation on new homes, written into the regulations rather than left to guidance. It is a generation requirement placed on the building, not a fabric requirement placed on your windows, and it creates no obligation to retrofit anything to an existing house.
You will also see the figure 1.2 W/m²K quoted alongside Future Homes coverage. That belongs to new dwellings under the Future Homes and Buildings Standard, and it does not come from this circular. It is not the number a replacement window in an existing home is measured against, and no installer should be quoting it at you as though it were. If you want the difference between the various U-value figures in plain English, our guide to U-values is the place to start.
The transitional protection, and who it is for
There is a grandfathering clause. For non-higher-risk work, and for work to an existing higher-risk building, the changes do not apply where a building notice, an initial notice, or an application for building control approval with full plans was given to the relevant authority before 24 March 2027, provided the building work commences before 24 March 2028. So a scheme already through the door before the commencement date carries a year to get started under the old rules.
That is a developer and self-builder provision. It matters if you have a new dwelling in the pipeline. It does not come into play when you are replacing windows in a house that is already built, because that work is certified under a different route entirely, which our guide to FENSA covers.
What we have not confirmed
Here is the honest gap, and we would rather print it than fill it with a guess. Circular 01/2026 does not mention windows, glazing or U-values at all. Its subject is the regulations framework, the L3 renewables requirement and the commencement dates. The detailed Approved Document L guidance that applies to existing dwellings sits outside this circular, and we have not read the 2026 edition of Approved Document L Volume 1 directly to see whether anything in it moves.
So we are not going to tell you the 1.4 W/m²K replacement floor is untouched, and we are not going to tell you it is about to tighten. The secondary write-ups we checked contradict each other on exactly this point, which is a good sign that none of them has read the document either. What we can say is what is sourced: 1.4 W/m²K, or Window Energy Rating band B, has applied to replacement windows since 15 June 2023 under Approved Document L, and Circular 01/2026 is a measure about new dwellings and new building work. When we can check the 2026 Approved Document L in full, we will update this page and say so in our double glazing news and policy updates.
Every time a Future Homes headline lands, my phone starts ringing with people convinced they have missed a deadline or that their quote is now illegal. New-build rules are not retrofit rules. If your house is already standing, ask the installer for the whole-window U-value in writing and for the registration certificate afterwards, and let the rest of it go.
Tom Bradley, window installer
What to do if you are replacing windows this year
Nothing about this circular should change the quote in front of you. Ask for the whole-window U-value rather than the centre-pane figure, because those are different numbers and only one of them is the one Approved Document L cares about. Any decent modern unit clears the replacement threshold without heroics, so a quote that leans on regulation as a reason to upsell you deserves a second look, and the tactics we set out in the coverage of the Warm Homes Plan apply just as well here. Policy noise is a favourite sales prop.
If you are budgeting, price the job before you worry about the legislation. Our uPVC window cost calculator will give you a range to sanity-check quotes against. And if you are weighing up whether to do the work at all, the more consequential change for existing homes is not this circular but the overhaul of how EPCs are calculated, because that one scores the fabric of the home you already own.
Frequently asked questions
This is a new-build standard. Building Circular 01/2026 covers new dwellings and new building work, and the new functional requirement it creates, L3, is for on-site renewable electricity generation for new dwellings and buildings containing dwellings. If you are replacing windows in a home that already exists, this circular is not the rule that governs your job.
The Building Regulations etc. (Amendment) (England) Regulations 2026 come into force on 24 March 2027. Regulations 3, 4, 6, 7, 8 and 9 come into force for higher-risk building work, and for work to existing higher-risk buildings, on 24 September 2027, subject to transitional provisions.
L3 is a new functional requirement for the provision of a system of on-site renewable electricity generation for new dwellings and buildings containing dwellings. It is about generating electricity on the building, not about glazing, and it does not create any obligation to retrofit an existing home.
We have not confirmed that either way, so we are not going to claim it. Circular 01/2026 does not mention windows, glazing or U-values at all. The 1.4 W/m²K maximum for a replacement window, or a Window Energy Rating of band B or better, has applied since 15 June 2023 under Approved Document L, and the detailed Approved Document L guidance for existing dwellings sits outside this circular.
For non-higher-risk work, and for work to an existing higher-risk building, the changes do not apply where a building notice, an initial notice, or an application for building control approval with full plans was given before 24 March 2027, provided the work commences before 24 March 2028. It matters if you have a new dwelling in the pipeline. It does not come into play when you are swapping windows in an existing house.
The instrument is titled The Building Regulations etc. (Amendment) (England) Regulations 2026, so it is an England measure. Building regulations are devolved, and Scotland, Wales and Northern Ireland set their own standards and timetables.