Part L 2026 sets the minimum thermal performance standard for replacement windows in England
The 2026 edition of Approved Document L (Conservation of Fuel and Power) applies to all replacement window installations in existing dwellings in England. The key change from previous editions is a tighter fabric energy efficiency standard that directly affects the maximum permissible U-value for new glazing. The single most important number a homeowner needs to know is the maximum centre-pane U-value for a replacement window: 1.6 W/m²K (Approved Document L, Volume 1: Dwellings, 2026 edition – Table 4.2, GOV.UK). This applies regardless of whether the window is double- or triple-glazed; the whole-window U-value, including the frame, must also meet the standard (typically ≤ 1.8 W/m²K, verified by the manufacturer’s certified data).
Part L 2026 requires a maximum centre-pane U-value of 1.6 W/m²K for replacement windows in England, down from 1.8 W/m²K in 2021. The whole-window limit is ≤ 1.8 W/m²K. Check manufacturer certification before installing.
- Maximum centre-pane U-value for replacement windows is 1.6 W/m²K (Part L 2026).
- Whole-window U-value must be ≤ 1.8 W/m²K, verified by manufacturer data.
- 2026 limit is 11% tighter than the 2021 standard of 1.8 W/m²K.
- Argon-filled double glazing with low-e coating typically meets the new standard.
- Triple glazing easily complies with the 1.6 W/m²K centre-pane limit.
- Part L 2026 sets the minimum thermal performance standard for replacement windows in England
- How the Part L 2026 U-value limit compares to the 2021 standard
- Quick numbers Part L 2026 window compliance at a glance
- The direct answer what "Part L windows" means for a 2026 replacement
- How to verify your window installer meets Part L 2026 and certification requirements
- What happens if your new windows do not meet Part L 2026 U-values
- How Part L 2026 interacts with other window-related grants and schemes
How the Part L 2026 U-value limit compares to the 2021 standard
The 2021 edition of Part L permitted a centre-pane U-value of up to 1.8 W/m²K for replacement windows. The 2026 limit of 1.6 W/m²K is a roughly 11% improvement in thermal performance (Approved Document L, Volume 1, 2021 edition – Table 4.2, GOV.UK). The whole-window U-value target has also tightened, moving from a typical maximum of 2.0 W/m²K (2021) to 1.8 W/m²K (2026) (DESNZ impact assessment for Part L 2026 update, published on GOV.UK). For a typical semi-detached house with 10 windows, upgrading from a 1.8 W/m²K to a 1.6 W/m²K centre-pane U-value can reduce annual heat loss through the glazing by approximately 10-12%, though total bill savings depend on the existing glazing and house fabric.
Quick numbers Part L 2026 window compliance at a glance
| Requirement | Part L 2026 (new standard) | Part L 2021 (previous standard) |
|---|---|---|
| Maximum centre-pane U-value (W/m²K) | 1.6 | 1.8 |
| Maximum whole-window U-value (W/m²K) | 1.8 | 2.0 |
| Typical argon-filled double glazing (low-e coating) – does it comply? | Yes, if certified ≤ 1.6 centre-pane | Yes |
| Typical triple glazing – does it comply? | Yes, typically achieves 0.8–1.2 centre-pane | Yes |
| Typical old (pre-2002) double glazing – does it comply? | No, typically 2.5–3.0 centre-pane | No |
Sources: Approved Document L 2026 Table 4.2, GOV.UK; Approved Document L 2021 Table 4.2, GOV.UK; MCS certified product data sheets, typical values for low-e argon double glazing and triple glazing.
The direct answer what “Part L windows” means for a 2026 replacement
For a homeowner replacing windows in 2026, “Part L compliant” means any new window must have a whole-window U-value of no more than 1.8 W/m²K and a centre-pane U-value of no more than 1.6 W/m²K. This is the single legal performance requirement under Building Regulations for the glazing itself in a replacement scenario. It is not an optional recommendation. Compliance is demonstrated by the window manufacturer providing a Certificate of Compliance or a product data sheet showing certified U-values from an accredited test lab (e.g., BFRC, CERTIF). The installer must not fit a window that does not meet these figures, and a Building Control Body (local authority or approved inspector) will check compliance as part of the overall work.
How to verify your window installer meets Part L 2026 and certification requirements
Any installer replacing windows in an England dwelling must be registered with a Competent Person Scheme (CPS) for windows, such as FENSA or CERTASS (GOV.UK page “Competent person schemes for building work”). The CPS registration allows the installer to self-certify compliance with Part L without needing a separate Building Control application. The homeowner receives a certificate of compliance. The installer must provide evidence that the specific window model they are fitting meets the 2026 U-value limits. Ask for the manufacturer’s product data sheet showing the whole-window U-value and centre-pane U-value. The installer must also install the window according to the manufacturer’s instructions to ensure the stated U-value is achieved (e.g., correct cavity fill, proper sealing). The CPS scheme will audit this.
how to check your FENSA certificate is valid
What happens if your new windows do not meet Part L 2026 U-values
Fitting a window that does not meet the 1.6 W/m²K centre-pane or 1.8 W/m²K whole-window U-value is a breach of Building Regulations (Building Act 1984, Section 35). The local authority can issue an enforcement notice requiring the window to be replaced or upgraded. Failure to comply can result in a fine (up to £5,000 per offence in a magistrates’ court, or unlimited in the Crown Court). If you sell the house later, the buyer’s solicitor will ask for Building Regulations compliance certificates. Without a FENSA or similar certificate, the sale can be delayed or fall through, or you may need to pay for retrospective indemnity insurance.
How Part L 2026 interacts with other window-related grants and schemes
The Energy Company Obligation (ECO4) scheme, which runs until March 2026, may fund replacement windows for eligible low-income households. From April 2026, the successor scheme (ECO5) will apply (Ofgem ECO4 guidance; DESNZ ECO5 consultation document). Under ECO4, replacement windows must meet a minimum whole-window U-value of 1.6 W/m²K, which is tighter than the Part L 2026 standard for replacement windows (Ofgem ECO4 Order, Schedule 2, Table 2). The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing. If you are applying for any grant, the installer must be registered with the relevant scheme (e.g., TrustMark for ECO4) and the window must meet the scheme’s specific U-value requirements, which may be stricter than Part L.
how to apply for ECO5 window replacement funding
Frequently Asked Questions
The maximum centre-pane U-value for replacement windows in England under Part L 2026 is 1.6 W/m²K (Approved Document L, Volume 1, Table 4.2, GOV.UK). The whole-window U-value, including the frame, must be ≤ 1.8 W/m²K.
Part L 2026 requires a centre-pane U-value of 1.6 W/m²K, down from 1.8 W/m²K in 2021 – an 11% improvement (GOV.UK Approved Document L). The whole-window limit also tightened from 2.0 W/m²K to 1.8 W/m²K.
Yes, Part L 2026 applies to all replacement window installations in existing dwellings in England (Approved Document L, Volume 1, 2026 edition, GOV.UK). The same U-value limits apply as for new-build windows.
Argon-filled double glazing with a low-e coating typically meets the 1.6 W/m²K centre-pane U-value if certified. Triple glazing also complies easily (Energy Saving Trust guidance). Always check manufacturer-certified data for compliance.
Yes, triple glazing easily meets the Part L 2026 centre-pane U-value limit of 1.6 W/m²K and typically performs better (MCS certified products). It may offer additional energy savings, but is not mandatory.
