Why we went back to the source
When we covered the Future Homes Standard regulations in August, we said we had not read the 2026 Approved Document L itself, so we would not tell you whether the replacement window standard had moved. The secondary write-ups disagreed. On 26 October 2026 we downloaded the current file from GOV.UK, the version revised on 8 September 2026, and read the window sections against the 2021 edition it replaces. This is what they say.
Replacement windows: no change
Table 3.2 of the 2026 edition sets the limits for new fabric elements in existing dwellings. For a window it reads "1.4 or Window Energy Rating Band B minimum", exactly as Table 4.2 of the 2021 edition did. Paragraph 3.9 repeats the second test, that a replacement must be no worse than the element it replaces. Glazed doors keep their own limits: 1.4 or a Doorset Energy Rating of band C where more than 60% of the face is glass, band B otherwise.
So the advice in our guide to Building Regulations Part L for windows stands. Any modern A-rated unit clears the bar, and an installer who tells you a new 2027 rule forces an upgrade is wrong about replacement work.
| Window rule | 2021 edition | 2026 edition |
|---|---|---|
| Replacement window, existing home | 1.4 W/m²K or WER band B | 1.4 W/m²K or WER band B (unchanged) |
| New-build window limit | 1.6 W/m²K | 1.6 W/m²K (unchanged) |
| How a new-build U-value is worked out | Actual size, standard 1.23m × 1.48m size, or hot-box | Actual size and configuration, or hot-box only |
| How an existing-home U-value is worked out | Actual size, standard size, hot-box or SAP default | Same options (unchanged) |
| Listed or historic building | 1.2 W/m²K centre pane, or low-E secondary glazing | Same (unchanged) |
The 1.6 W/m²K new-build figure is a backstop for each window. A new home must also meet whole-dwelling targets set against a notional dwelling, so its windows are judged as part of the whole house too.
New homes: the actual size rule
The change is in paragraphs 3.2 and 3.3, and it only applies to new dwellings. Under the 2021 edition, any window's U-value could be calculated for a standard window 1.23m wide and 1.48m high, whatever the real window measured. From 24 March 2027, a new-build window has to be either measured by the hot-box method or calculated using its actual size and configuration.
The 2026 text lists what that calculation must include: the actual frame sections, sills and thresholds, frame extenders, dummy sashes, dummy transoms and mullions, decorative Georgian bars inside the cavity, and non load-bearing couplers such as a PVC H-coupler. Load bearing items such as bay poles are left out. Veka's national specification manager, writing in Window News in October, summed up the point: a headline range figure no longer tells the whole story for a new home.
For homeowners the takeaway is narrower. The figure in a brochure is often a standard-size figure, and the window on your wall is not standard. Our guide to U-values explains the difference between centre-pane and whole-window numbers, which is often the bigger gap on a quote.
Listed buildings, conservation areas and glass-only jobs
Paragraph 3.11 keeps the exception for buildings whose character has to be kept. Where windows cannot meet Table 3.2 for that reason, they can meet a centre pane U-value of 1.2 W/m²K instead, or existing single glazing can be supplemented with low-emissivity secondary glazing. If that is your situation, our listed building window cost guide and the secondary glazing cost guide cover the options.
Paragraph 10.6 is unchanged in effect too. Replacing the glass, or a window within its existing frame, is not providing a controlled fitting and does not have to meet the energy efficiency requirements. That is why a blown sealed unit can be swapped without a FENSA certificate, while a full window replacement needs one, as our guide to FENSA sets out.
The useful news here is a non-event. If your house is already built, nothing in the 2026 document changes what your replacement windows have to meet, so there is no deadline to beat and no new rule to pay for. Ask for the whole-window U-value or the WER band in writing, and check it clears 1.4 or band B.
Double Glazing Cost Calculator Team
What to do next
Price the job on today's rules, because they are also 2027's rules for replacement work. The uPVC window cost calculator gives a range for your house, the Window Energy Rating guide explains the bands on a quote, and you can get quotes from local installers when you are ready.
Frequently asked questions
No. Table 3.2 of the 2026 edition keeps the limit for a new or replacement window in an existing home at a U-value of 1.4 W/m²K or a Window Energy Rating of band B or better, the same as the 2021 edition. The replacement must also be no worse than the window it replaces.
On 24 March 2027 for building work in England that is not connected with higher-risk building work, and on 24 September 2027 for higher-risk building work. It was first published on 24 March 2026 and revised on 8 September 2026 to correct minor errors. Transitional arrangements are in Circular Letter 01/2026.
For new dwellings, a window's U-value must be measured by the hot-box method or calculated using the window's actual size and configuration, including frame sections, sills, frame extenders, dummy sashes, dummy mullions and transoms, Georgian bars inside the cavity and couplers. Bay poles are left out. The 2021 edition let new-build windows use a standard 1.23m by 1.48m size instead.
No. For existing dwellings the 2026 edition still allows a U-value calculated at the standard 1.23m by 1.48m size, with the window's own configuration or a standard one, as well as actual size, the hot-box method or the SAP 10.3 default values.
The 2026 edition keeps the exception. Where windows cannot meet Table 3.2 because the character of the building has to be kept, they can instead meet a centre pane U-value of 1.2 W/m²K, or single glazing can be supplemented with low-emissivity secondary glazing.
No. Paragraph 10.6 says replacing the glazing, or a window in its existing frame, is not providing a controlled fitting and does not need to meet the energy efficiency requirements. The rules bite when the whole unit, frame included, is replaced.