Window U-Values UK: 1.4, or Window Energy Rating Band B
A replacement window has to pass two tests, not one, and the timber concession that installers still quote expired in June 2023. What Approved Document L actually sets, and what the 2026 edition changes.
Replacement window: maximum 1.4 W/(m2·K), or Window Energy Rating Band B.
And: it must also be no worse than the window it replaces. Both tests apply.
Doors: 1.4, or Doorset Energy Rating Band C if more than 60% of the internal face is glazed, Band B otherwise. Rooflights: 2.2.
2026 edition: in force 24 March 2027, and it keeps windows at 1.4 or Band B. This number is not about to move.
Window U-values in the UK are set by Part L of the Building Regulations, and the practical guidance sits in Approved Document L Volume 1, which covers dwellings. Almost every quotation for replacement windows mentions a U-value or a Window Energy Rating band somewhere in the small print, and almost none of them explain that a window has to clear two separate hurdles before building control will accept it.
This is what the document requires, and where quotations tend to go wrong.
The two tests
Paragraph 4.8 of Approved Document L says the U-value of a replacement fabric element in an existing dwelling should both:
- be no worse than that of the element being replaced, and
- meet the limiting standard in Table 4.2.
The second test is the famous one. The first test is the one that catches renovations of houses that were already improved. If a previous owner fitted good triple glazing at 1.0, you cannot replace it with a 1.4 unit and call that compliant, even though 1.4 is the published limit. Like for like is the floor, not the target.
Table 4.2, in full
| Element | Maximum U-value, W/(m2·K) |
|---|---|
| Roof | 0.15 |
| Wall | 0.18 |
| Floor | 0.18 |
| Swimming pool basin | 0.25 |
| Window, including roof windows and curtain walling | 1.4, or Window Energy Rating Band B minimum |
| Rooflight | 2.2 |
| Door with more than 60% of its internal face glazed | 1.4, or Doorset Energy Rating Band C minimum |
| Other doors | 1.4, or Doorset Energy Rating Band B minimum |
Two notes on that table are worth knowing. Rooflight U-values are calculated on the outer developed surface area, which is usually larger than the roof opening, so a rooflight can perform worse in the numbers than the same glass would in a wall. And an external fire doorset, as defined in Appendix A of Approved Document B Volume 1, is allowed up to 1.8.
The timber concession expired in June 2023
When the 2021 edition landed, note 9 of Table 4.2 gave timber windows a transitional allowance: 1.6 W/(m2·K), or Window Energy Rating Band C, "permissible until 14 June 2023". Note 13 did the same for timber doors at 1.8, or Doorset Energy Rating Band E. Both notes then say the full standard applies from 15 June 2023.
Those concessions are more than three years dead. A joinery quotation that still offers timber casements at 1.6, or a timber front door at 1.8, is working from an arrangement that has expired. It is a useful thing to ask about, because it is checkable in one line of a specification and it tells you how current the supplier's technical information is.
The heritage route, which almost nobody offers
Paragraph 4.10 covers the case where windows or fully glazed external pedestrian doors cannot meet Table 4.2 because of the need to maintain the character of the building. It gives two alternatives, and only one of them has to be met:
- the fittings should not exceed a centre pane U-value of 1.2 W/(m2·K), or
- single glazing should be supplemented with low-emissivity secondary glazing.
Centre pane is a much easier target than a whole-window U-value, because it ignores the frame and the edge of the glass. It is what makes slim double glazed units in original timber sashes workable in a conservation area, and it is why secondary glazing is a compliant answer rather than a compromise. Very few installers volunteer this paragraph, and in a listed building or a conservation area it is often the difference between a scheme that gets consent and one that does not. It sits alongside, not instead of, the separate planning questions covered in our guides to listed building alterations and renovating in a conservation area.
What changes in the 2026 edition
Approved Document L Volume 1, 2026 edition, was published in 2026 and takes effect on 24 March 2027 for building work not connected with higher-risk building work, and on 24 September 2027 where it is. Transitional arrangements are set out in Circular Letter 01/2026.
For glazing, the answer is reassuring: Table 4.2 becomes Table 3.2, and the numbers do not move. Windows stay at 1.4 or Window Energy Rating Band B. Doors stay at 1.4 with Doorset Energy Rating Band C or Band B depending on the glazed area. Rooflights stay at 2.2. The two tests survive as paragraphs 3.8 and 3.9, and the character route survives as paragraph 3.11, now cross-referenced to paragraph 0.9.
So a window specified to 1.4 today is not about to be obsolete. If you are planning a renovation that will run into 2027, the glazing spec is one of the few parts of it you do not need to hedge.
The certificate you actually get
There is no such thing as a FENSA certificate, whatever the paperwork says. Replacement windows and doors are controlled fittings, so the work is notifiable. The legal document is a Building Regulations Compliance Certificate, and it can be issued by any government-authorised competent person scheme, of which FENSA and Certass are two, or obtained through a building control application if the installer is not registered.
That matters at the point of sale. A conveyancer asks for the certificate for any window replaced since April 2002, and an indemnity policy is a worse answer than a certificate. Ask which scheme your installer is registered with before the deposit, not after the fit.
How to read a window quotation
- Look for a whole-window U-value, not a centre pane figure, unless you are using the paragraph 4.10 heritage route. Centre pane numbers look far better and are not the compliance test.
- A Window Energy Rating band is an acceptable alternative to a U-value, and building control bodies may accept a rating from a quality assured certification scheme as evidence.
- If your existing windows are already better than 1.4, say so in writing, because the replacements have to match them.
- Check the scheme registration and get the compliance certificate number when the job is signed off.
Frequently Asked Questions
What U-value do replacement windows need in the UK?
A maximum of 1.4 W/(m2·K), or a Window Energy Rating of Band B, under Table 4.2 of Approved Document L Volume 1. The replacement must also be no worse than the window being replaced, so an already-efficient window sets a higher bar than 1.4.
Is 1.4 the same for timber, uPVC and aluminium windows?
Yes. A transitional note allowed timber windows 1.6, or Band C, until 14 June 2023, and timber doors 1.8, or Doorset Energy Rating Band E, on the same date. From 15 June 2023 the full 1.4 standard applies to every frame material.
What is the rule for windows in a listed building or conservation area?
Paragraph 4.10 applies where the character of the building prevents meeting Table 4.2. You then either keep the fitting to a centre pane U-value of 1.2 or supplement single glazing with low-emissivity secondary glazing. Planning and listed building consent are separate questions and still apply.
Do the U-value rules change in 2027?
Not for glazing. The 2026 edition of Approved Document L Volume 1 takes effect on 24 March 2027, and its Table 3.2 keeps windows at 1.4 or Window Energy Rating Band B, doors at 1.4 and rooflights at 2.2.
What U-value does a new external door need?
1.4, expressed either as a U-value or as a Doorset Energy Rating: Band C where more than 60% of the internal face is glazed, Band B for other doors. External fire doorsets are allowed up to 1.8.
Do I need building control approval to replace a window?
Yes. Windows and doors are controlled fittings and the work is notifiable. It is normally handled by an installer registered with a competent person scheme such as FENSA or Certass, who then issues a Building Regulations Compliance Certificate. Otherwise you make a building control application.
Sources
- GOV.UK: Conservation of fuel and power, Approved Document L, Volume 1 (2021 edition incorporating 2023 amendments), Table 4.2 and paragraphs 4.8 and 4.10
- Approved Document L Volume 1, 2026 edition, Table 3.2 and paragraphs 3.9 and 3.11
More from Vertigo Interiors: our building regulations guide for London renovations and the period property design guide.
Figures checked on 7 September 2026 against the published PDFs of Approved Document L Volume 1, 2021 edition incorporating 2023 amendments, and the 2026 edition. This is general guidance, not a substitute for advice from your building control body.