New window rules in 2026 came into force on 15 June 2026
If you are planning to replace windows or build a new home in 2026, the rules changed on 15 June. The 2026 update to Part L of the Building Regulations for England took effect on that date, as confirmed by the Ministry of Housing, Communities and Local Government in its 2025 consultation response document “The Future Homes Standard: 2025 consultation response” (MHCLG, 2026). The rules apply to all new window installations in both new-build homes and replacement windows in existing dwellings.
Wales and Scotland operate separate building standards, and Northern Ireland follows its own timeline for equivalent changes. The key change is a tighter target U-value for windows: 1.2 W/m²K for new-build and 1.4 W/m²K for replacement windows in existing homes (DESNZ, 2026).
The direct answer building regulations windows 2026 require U-values of 1.4 W/m²K for replacements and 1.2 W/m²K for new-build
For homeowners replacing windows in an existing home, the legal maximum U-value is 1.4 W/m²K. For new-build homes, it is 1.2 W/m²K. These targets apply to all window installations in England from 15 June 2026, under Part L of the Building Regulations.
Triple glazing is effectively mandatory for new-build and strongly recommended for replacements. High-performance double glazing may meet the replacement target, but you should check with your installer. The rules are built to cut heat loss through windows by roughly 12–15% compared to the previous standard. This contributes to overall home energy efficiency improvements (DESNZ, 2026).
The new maximum U-value for replacement windows in existing homes is 1.4 W/m²K
Previous Part L (2013 edition) allowed replacement windows up to 1.6 W/m²K. The 2026 update reduces this by 0.2 W/m²K (DESNZ, 2026). U-value measures heat loss through the window: lower numbers mean better insulation. 1.4 W/m²K is now the legal maximum for any replacement window installed in an existing home in England.
Triple glazing is not mandatory but is typically required to achieve 1.4 W/m²K. High-performance double glazing with low-E coatings and argon fill may also meet the target. The rule applies to all window types: casement, sash, tilt-and-turn, and fixed lights. Rooflights have a separate target of 1.6 W/m²K (DESNZ, 2026).
New-build homes must meet a 1.2 W/m²K U-value for windows
New-build homes under the 2026 rules face a stricter 1.2 W/m²K target, down from 1.4 W/m²K under the 2021 edition (DESNZ, 2026). This target is part of the Future Homes Standard, which mandates overall fabric energy efficiency improvements of roughly 20–30% compared to 2021 standards.
Achieving 1.2 W/m²K virtually always requires triple glazing with thermally broken frames and warm-edge spacers. The rule applies to all new dwellings, including self-builds and custom homes, from 15 June 2026 onward.
Quick numbers U-value targets compared
| Window type | Pre-2026 maximum U-value (W/m²K) | 2026 maximum U-value (W/m²K) | Change (W/m²K) |
|---|---|---|---|
| New-build new home | 1.4 | 1.2 | -0.2 |
| Existing home replacement | 1.6 | 1.4 | -0.2 |
| Rooflight new-build | 1.8 | 1.6 | -0.2 |
| Rooflight replacement | 2.0 | 1.6 | -0.4 |
Source for all figures: DESNZ, 2026, Tables 4.2 and 4.3.
You do not need to replace existing windows immediately
The 2026 rules apply only to new installations, not to windows already fitted before 15 June 2026. Existing windows are exempt from retrospective upgrade (MHCLG, 2026, paragraph 4.12). If you replace a single window or a whole set, each replacement must meet the 1.4 W/m²K standard for existing homes.
If you repair rather than replace (for example, replace a broken pane or sash cord), the U-value rule does not apply, provided the frame remains and the glass is like-for-like. Planning permission is not required for replacement windows unless the property is listed or in a conservation area, but Building Regulations approval is mandatory.
How to verify your installer meets the 2026 standards
All installers must provide a certificate of compliance with Building Regulations for new window installations. This is typically done via FENSA or CERTASS self-certification schemes for replacement windows (MHCLG, 2026, section 8). For new-build homes, windows must be tested or certified by a UKAS-accredited laboratory to confirm the stated U-value. Ask for the manufacturer’s BFRC rating certificate.
Check that your installer is registered with FENSA, CERTASS, or a similar Competent Person Scheme. Unregistered installers require a separate Building Control application from the local authority. TrustMark registration is not mandatory for window installers but is a voluntary quality mark. MCS certification applies only to solar control glazing, not standard window replacement.
