After years of consultation and delay, the government published the 2026 edition of Approved Document L alongside the Future Homes Standard on 24 March 2026. The new rules come into force in England on 24 March 2027 for standard building work, and on 24 September 2027 for work connected with higher risk buildings. Wales is moving on a comparable timetable. That leaves the industry a transition of about a year, noticeably shorter than the run up to the 2022 changes.
Having watched every Part L revision since the 2000s, I can tell you the Future Homes Standard for windows and doors is different in character. Most coverage leads with the U-values, and on that measure very little has changed. The notional target for new build windows stays at 1.2 W/m²K. The story sits elsewhere. It is in how performance must now be calculated, evidenced and installed. For anyone specifying, buying or fitting windows and doors, those are the changes that matter.
Future homes Standard for Windows and Doors. The end of the ‘standard’ window
For years the industry has declared window U-values using a single reference size of 1230mm by 1480mm. One calculation on one convenient size, and that figure went onto every quotation regardless of what was actually installed. A small casement, a large fixed light and a door with a midrail all sold under the same number.
Anyone who has run thermal calculations knows this flattered some products and understated others. Small windows carry proportionally more frame and perform worse than the standard size suggests. Large glazed areas often perform better. The declared figure and the installed reality frequently had little to do with each other.
From 2027, that approach is no longer acceptable for new dwellings. Future Homes Standard for windows and doors means U-values must be calculated using the actual size and configuration of each individual unit, in line with the relevant BS EN ISO standards, or measured directly by hot box testing. The results are then combined into an area weighted average across the dwelling. This is the government closing the gap between what is claimed on paper and what is delivered on site, and in my view it is overdue.
Future Homes Calculations. What must now be counted, and what must not
The bigger shift is what the calculation has to include. Until now, frames were treated as a simple element and a great deal was quietly left out. From 2027, thermal calculations for new dwellings must account for the parts that genuinely affect performance:
- Frame sections and their variants
- Cills and thresholds
- Add-ons and frame extenders
- Dummy sashes
- Dummy transoms and mullions
- Decorative bars inside the insulated glass unit cavity
- Non-structural couplers
What you do not include are dead load bearing components such as bay poles.
The practical effect is that every design decision now has a thermal consequence that must be proved. Add a coupler to join two frames, drop in a cill extension, add astragal bars for a heritage look, and each one has to be carried through the calculation. The convenient brochure figure is finished for new build work.
Notional, limiting, and the England and Wales divide
Two numbers matter and they are routinely confused. The notional U-value is the design starting point, and for windows it stays at 1.2 W/m²K. The limiting U-value is the worst any single element is allowed to reach, on the basis that other parts of the building make up the difference.
Here England and Wales part company. In England the limiting U-value for new build windows and glazed doors is 1.6 W/m²K. In Wales it is 1.4 W/m²K. Wales has taken the tighter line. If you are specifying across both nations, that distinction is not academic. It changes what a single window is allowed to do before the rest of the fabric has to compensate.
For existing dwellings the position is more relaxed and largely unchanged. Standard size calculations remain acceptable. Replacement windows still need to achieve 1.4 W/m²K or a Window Energy Rating of Band B. Replacement doors with more than 60 per cent glazing need 1.4 W/m²K or a Doorset Energy Rating of Band C, and other doors need 1.4 W/m²K or Band B. A replacement unit must also be no worse than the element it replaces.
The Good News, Glazing no longer sits in isolation
One consequence of the new approach is easy to miss. Windows and doors can no longer be specified purely on their own U-value, because the amount and position of glazing now feeds into two other parts of the regulations. Part O governs overheating, and Part F governs ventilation. Both are affected by how much glass you put in a wall and where you put it.
This matters most for the large glazed areas so many people want, particularly on south facing elevations. A big run of glass can help the fabric sum on paper while pushing the design into overheating risk under Part O, at which point you are into solar control glass, shading or reduced opening sizes to bring it back into line. Ventilation provision under Part F has to be resolved at the same time. The point is that glazing ratios are now a whole building decision taken early with the installer and designer together, not a product choice made in isolation at the end. If your heart is set on a wall of glass, raise it at design stage so it can be made to work rather than discovered as a problem later.
Installation now carries as much weight as fabrication
Part L now puts real emphasis on how windows and doors are fitted, not just how they are made. A well made window installed badly is a non compliant window.
On airtightness, installers must seal frames to the primary air barrier with continuous seals around the full perimeter. Penetrations around fixings, cables and packers must be sealed with the appropriate tapes, membranes, grommets or sealants. Frames must be taped or sealed to the surrounding structure so air cannot leak around the opening.
The whole dwelling has to meet an airtightness target of 8.0 m³/(h·m²) at 50 Pa, and windows and doors are a common weak point in reaching it. Poor work here can cause the dwelling to fail its pressure test, which means going back and putting it right.
This is the point where a good survey and a competent fitting team stop being a nice extra and become a compliance requirement.
The Paperwork is now part of your windows and doors
Building control now expects a proper audit trail. In 2021 a product sheet often did the job. That era is over. For new dwellings you can expect to provide configuration specific U-value calculations at the actual installed sizes, the full product specifications used in the thermal model, and photographic evidence showing the insulation and airtightness measures around each opening before they are covered up.
Those photographs are referenced in the compliance report, and the installer must hand operation and maintenance information to the homeowner within five days of completion.
If you are a homeowner, this matters to you directly. The evidence pack is what proves your new windows actually meet the standard. Ask who is responsible for producing it.
Replacement Windows , refurbishment and extensions
If you are replacing windows or doors in an existing home, the headline changes from 2027 alter very little for you today. The limiting values above still apply, standard size declarations are still accepted, and the unit must not worsen the thermal performance of what it replaces. It must also be reasonably draught proof.
Extensions are treated differently. New windows and doors installed into an extension must meet the same limiting values as any other new element, so an extension is closer to new build territory than a straight replacement.
Heritage and listed buildings
There is sensible flexibility for older buildings. Listed buildings and properties in conservation areas do not have to comply fully where doing so would unacceptably alter the character or appearance of the dwelling. Where improvements are made, a centre pane U-value of 1.2 W/m²K or well specified secondary glazing are treated as acceptable approaches. If you own a period property, this is worth understanding before anyone tells you that you are obliged to strip out original windows. Often you are not.
Future Homes Standard For Windows and Doors. Competence is now mandatory
One of the quieter but more significant changes concerns who is allowed to do the work. In 2021 competency was not explicitly required. Now it is. Installers and fabricators must be able to demonstrate appropriate skills, knowledge and experience, deliver compliant workmanship with the documentation to back it, and hold Competent Person Scheme registration where they are self certifying the work.
For buyers this is genuinely good news. It raises the quality standard. It also gives you a fair question to ask any company quoting for your job.
Ready to buy new windows and doors? What Future Homes Standard means if you are buying now
Take the pressure selling with a pinch of salt. The Future Homes Standard is aimed squarely at new dwellings. If a salesperson tells you the regulations are forcing you into triple glazing for a straightforward replacement today, they are overstating the position. Triple glazing has real merits in the right application, but a new build regulation is not the reason to buy it.
That said, the direction of travel is clear. The move to actual size calculation started with new build, and everything about it suggests it reaches the replacement market in time. When you are spending serious money on windows and doors that should last twenty five years or more, buy products whose real performance, at your sizes and in your configurations, comfortably clears the minimum rather than scraping it.
A few questions separate the businesses that understand their products from the ones that do not. Ask how the U-value was determined, at what size, and whether it includes any couplers, cills or bars in your particular design. Ask whether the figure is a whole window value rather than a centre pane value, a distinction that still catches people out and one I have been correcting in sales literature for decades. Ask who produces the compliance evidence and hands it over at the end. And ask whether the installer is on a Competent Person Scheme.
The questions are the easy part. Grading the answers is harder, and it is where most people come unstuck. A supplier can talk fluently and still be quietly dodging the point. If you have put these questions to a company and you are not sure whether the answers hold up, that is exactly the moment to get an independent view before you sign anything.
The new rules reward companies that know their products properly and can prove what they claim. They expose those that cannot. For buyers who ask the right questions, that is a good position to be in.
Talk to us before you choose an installer
The new rules reward companies that understand their products and can prove it, and they expose the ones that cannot. The trouble is that both types quote confidently, and from a homeowner’s chair they can be hard to tell apart until the evidence is put to the test.
That is where we can help. Tell us about your project and we will point you towards installers who genuinely understand the Future Homes Standard, can calculate a real U-value at your sizes, and will produce the compliance evidence properly at the end. And we will include a free pdf, with questions to ask your installer to demonstrate compliance. Or contact us for your copy.
Door and Window Experts has been giving free and independent advice on upvc, timber, steel and aluminium windows and doors since the early days of the internet, drawing on industry experience going back to 1989, with more than 90 five star reviews from people we have helped. If you are planning a new build, an extension or a major glazing purchase, get in touch before you commit. Putting you in front of the right installer is exactly what this website exists for.
Further Reading:
Building Regulations for Windows and Doors
Part L of Building Regulations
Competent Person Schemes
