The target U-value for new windows in the 2026 Building Regulations
The 2026 edition of the Building Regulations, specifically Part L covering the conservation of fuel and power, sets legally binding maximum U-values for all new and replacement windows. For replacement windows in existing homes, the whole-window U-value must not exceed 1.4 W/m²K (GOV.UK Approved Document L, 2025 amendments). For new-build homes, the requirement is tighter at 1.2 W/m²K. These figures apply to the complete window unit, including the frame, glass panes, and the edge seals, not just the centre-pane glass value. The regulations came into effect in June 2025, so any installation from that date onward must comply.
How a WER rating translates to a U-value
A Window Energy Rating (WER) is a banded score from A++ to G, calculated by the British Fenestration Rating Council (BFRC) using three factors: the U-value, the solar heat gain coefficient (g-value), and the air leakage rate. In 2026, a WER of A typically corresponds to a whole-window U-value of around 1.2 W/m²K or better (BFRC technical documentation, 2026). A WER of C or D is common for older double glazing installed before 2010, with a U-value of roughly 1.6–2.0 W/m²K. The WER band is a more comprehensive measure than the U-value alone, but U-value remains the primary regulatory target.
Quick numbers U-values, WER bands, and annual heat loss
| WER Band | Whole-window U-value (W/m²K) | Typical annual heat loss per m² (kWh) | Equivalent annual cost per m² (gas heating, £) |
|---|---|---|---|
| A++ | ≤0.8 | 35 | £3.60 |
| A | 1.2 | 52 | £5.36 |
| B | 1.4 | 61 | £6.28 |
| C | 1.6 | 70 | £7.21 |
| D | 1.8 | 79 | £8.14 |
| E | 2.0 | 88 | £9.06 |
Annual heat loss is calculated using the Energy Saving Trust’s standard degree-day method (1800 degree-days) and the January 2026 gas price of 10.3p/kWh from the Department for Energy Security and Net Zero quarterly energy price bulletin (DESNZ, October 2025). The difference between a D-rated window and an A-rated window is roughly £2.78 per square metre per year.
Who qualifies for the Great British Insulation Scheme or ECO4 window grant in 2026
The Great British Insulation Scheme (GBIS) and the Energy Company Obligation (ECO4) provide partial funding for window replacement, but only under strict conditions. The existing windows must be single glazing or very inefficient double glazing with a U-value above 1.8 W/m²K (Ofgem ECO4 guidance, 2025/2026). Eligibility is means-tested: the household must receive a qualifying benefit such as Pension Credit, Universal Credit, or Income Support, or live in a low-income area defined by the government’s Index of Multiple Deprivation. The property must also have an Energy Performance Certificate (EPC) rating of D, E, F, or G, and the window replacement must be part of a broader package of measures, such as loft or cavity wall insulation.
Who does NOT qualify for window grants in 2026
Homeowners whose windows already meet a U-value of 1.4 W/m²K or better are ineligible for any government-funded window grant. Households with an EPC rating of A, B, or C are excluded from both GBIS and ECO4 (Ofgem GBIS scheme guidance, 2025). Landlords and buy-to-let investors cannot access GBIS or ECO4 funding for window replacement at all. Also, second homes and properties that are not the main residence are not covered.
How to confirm your window installer is certified for grant-funded work
All window installations funded by ECO4 or GBIS must be carried out by an installer registered with the Microgeneration Certification Scheme (MCS) or a TrustMark-approved installer with a specific window competency (MCS installer database, 2026). For non-grant work, the installer should be FENSA or CERTASS registered to self-certify compliance with Building Regulations. Check the MCS installer database at mcscertified.com or the TrustMark website at trustmark.org.uk for the company’s current certification before any work begins. how to find a certified window installer in the UK
The direct answer to “What is the window U-value for UK 2026?”
For replacement windows in existing homes, the maximum whole-window U-value allowed under the 2026 Building Regulations is 1.4 W/m²K. For new-build homes, the maximum is 1.2 W/m²K. To qualify for a WER rating of A or higher, the whole-window U-value must be 1.2 W/m²K or better (Approved Document L, 2025 amendments; BFRC WER band definitions, 2026). If you are replacing windows, aim for at least a B-rated unit with a U-value of 1.4 W/m²K to meet the legal minimum.
What happens if your existing windows have a U-value above the 2026 standard
There is no legal requirement to upgrade existing windows unless you are replacing them under a Building Regulations application. If you sell your home, the EPC assessor will record the window U-value or WER from the manufacturer’s data or a standard assumption table used in the RdSAP 10.2 methodology (MHCLG EPC methodology, RdSAP 10.2, 2025). An EPC recommendation for window replacement can reduce the property’s energy rating, but it does not force replacement. how EPC ratings affect window replacement decisions The practical effect is that older windows with U-values above 1.4 W/m²K will increase your heating bills, but you are not required to change them until you choose to replace them.
Frequently Asked Questions
The target whole-window U-value for replacement windows in existing homes is 1.4 W/m²K, as set by the 2026 Building Regulations Part L (GOV.UK Approved Document L, 2025 amendments). For new-build homes, the requirement is 1.2 W/m²K.
A WER band A typically corresponds to a whole-window U-value of around 1.2 W/m²K or better in 2026, according to BFRC technical documentation. A C or D rating equates to roughly 1.6–2.0 W/m²K, common in older double glazing.
The difference is about £2.78 per square metre per year, based on the DESNZ January 2026 gas price of 10.3p/kWh. An A-rated window costs around £5.36 per m² annually, while a D-rated window costs £8.14 per m².
Yes, all replacement windows in existing homes must comply with the 2026 Building Regulations Part L, which sets a maximum whole-window U-value of 1.4 W/m²K. Installations from June 2025 onward must meet this standard, as confirmed by GOV.UK.
The best WER rating is A++, which corresponds to a whole-window U-value of ≤0.8 W/m²K and annual heat loss of just 35 kWh per m². This is the most energy-efficient option available under the BFRC rating system for 2026.
