Every limiting value for a controlled fitting
| Fitting | Limit (W/m²K) | Alternative / note |
|---|---|---|
| Window (replacement, existing dwelling) | 1.4 | or Window Energy Rating band B |
| Roof window in the plane of the roof | 1.4 | assessed in the vertical position |
| Rooflight on an upstand | 2.2 | assessed in the horizontal position |
| External door (≤ 60% glazed) | 1.4 | or Doorset Energy Rating band B |
| External door (> 60% glazed) | 1.4 | or Doorset Energy Rating band C |
Table 4.2, Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England). The next edition takes effect for work starting on or after 24 March 2027 — worth knowing if you are specifying for a job that starts around then.
Trap one: whole-window, not centre-pane
Glass units are marketed on their centre-pane U-value, which ignores the frame and the spacer entirely. Part L wants the whole-window figure — the number for the complete unit as installed, to BS EN ISO 10077 or via a WER certificate. The gap is not academic: a unit quoted at 1.1 centre-pane can be 1.5 whole-window, which fails. If a quotation gives you one number and no label saying which it is, that is the question to ask before signing.
Trap two: the small window
Frames conduct more heat than the glazed unit, so a window's U-value gets worse as it gets smaller — more frame per square metre. Manufacturers quote at a standard test size (commonly 1.23 × 1.48 m). The same product in a small bathroom or landing opening can miss the limit even though the brochure passes. Where a job is marginal, ask for the figure at the sizes you are ordering — we wrote that up when the 2026 edition's window and doorset values were published: why smaller windows perform worse.
Rooflight or roof window?
They are not the same thing under Part L, and the difference is nearly a whole W/m²K. A roof window sits in the plane of the roof and is treated as a window: 1.4 W/m²K, assessed in the vertical position. A rooflight on an upstand or kerb is assessed in the horizontal position and takes 2.2 W/m²K. Comparing a product's declared figure against the wrong one of those is a common way to fail a check that should have passed — the full explanation is here.
Listed buildings and conservation areas
Approved Document L carries an explicit exception where meeting the requirements would unacceptably alter the character or appearance of the building. It applies to listed buildings, buildings in conservation areas and those in an article 4 direction area, among others. It is not a blanket exemption — the expectation is that you improve energy efficiency as far as is reasonably practicable without that harm, which in practice means slim double glazing, secondary glazing, or a like-for-like single-glazed unit where the frames themselves are the heritage asset. What matters for Building Control is that the reason is recorded rather than the row quietly dropped, which is how RenoMonkey handles it.
Notification: FENSA, CERTASS or a building notice
Replacement windows and doors are notifiable work. Either your installer is registered with a Competent Person Scheme and self-certifies — that is what a FENSA or CERTASS certificate is — or the work goes to the building control body as a building notice. The U-value duty is identical on both routes; only the paperwork differs. Keep the certificate: it is one of the first things a buyer's solicitor asks for.
Replacing windows as part of bigger works
Two neighbouring rules are worth keeping straight. If the windows are going into a new extension, they are new elements and they also eat into the extension's 25% opening-area allowance — a separate check with its own consequences. If the wall around them is being renovated at the same time — new render, internal dry-lining, cavity fill over more than half the elevation — then the wall has its own Table 4.3 duty alongside the fittings. RenoMonkey takes both in the same project and prints one report.
Frequently asked questions
What U-value do replacement windows have to meet?
1.4 W/m²K for the whole window, or Window Energy Rating band B as the alternative route. Replacement doors take 1.4 W/m²K or Doorset Energy Rating band B — band C where more than 60% of the door is glazed. These are the Table 4.2 limiting values for controlled fittings in an existing dwelling.
Is that the glass U-value or the whole window?
The whole unit — frame, glass, spacer and all, to BS EN ISO 10077 or a WER. It is the single most common mistake on a quotation: a centre-pane figure of 1.1 or 1.2 looks compliant and the whole-window value for the same product can be 1.5. Ask the supplier for the whole-window U-value, and check it is for the size you are actually buying.
Do small windows perform worse?
Usually, yes, and it surprises people. Frame conducts more heat than the glazed unit does, so the smaller the window the higher the frame-to-glass ratio and the worse the whole-window U-value. A product quoted at a standard test size can miss the limit at a small bathroom or landing size.
Do I need Building Control for replacement windows?
Either the installer is registered with a Competent Person Scheme such as FENSA or CERTASS and self-certifies the work, or the job is notified to the building control body. Both routes require the same U-value; the scheme route just avoids a separate application. Keep the certificate — it is what a buyer's solicitor asks for.
My house is listed / in a conservation area — do the limits still apply?
There is an explicit exception where meeting the energy efficiency requirements would unacceptably alter the character or appearance of the building — listed buildings, buildings in conservation areas, and those in an article 4 direction area among them. It is not a blanket exemption: the work should still improve energy efficiency as far as is reasonably practicable without that harm, and the reason should be recorded. RenoMonkey keeps the exception on the report rather than quietly dropping the row.
What about windows in an extension?
Different table, same number. Windows in a new extension are new elements under Table 4.2, and they also count towards the 25% opening-area allowance for the extension — which is a separate check with its own consequences. That one is GlazeMonkey's side of the same tool.
Does replacing windows trigger anything else?
It can trigger a ventilation duty: replacing windows should not make the ventilation of the room worse, which is why trickle vents so often appear in a replacement quote. And if a window is being made bigger or a new opening formed, structure and Part L both come into it. The U-value is the part this tool covers.
Next: renovating walls, roofs and floors · U-values for a new extension · the dated reference tables.