Yes, double glazed windows should have a kite mark, specifically the BSI Kitemark or equivalent UKAS-accredited certification, to prove they meet British Standards for safety, thermal performance and durability. The government’s competent person schemes and building regulations require windows to achieve a minimum U-value of 1.6 W/m²K for replacement installations (GOV.UK, Approved Document L, 2026). A kite mark is the easiest way to confirm compliance without a separate thermal test.
Not all double glazed windows need a kite mark, it depends on how they are installed. If a FENSA-registered or CERTASS-registered installer fits the windows, they self-certify compliance with building regulations, and no separate kite mark is mandatory. However, for DIY installations or windows bought online without a certified installer, a kite mark provides essential proof that the product meets the required standards. Without it, you risk windows that could fail thermal efficiency tests or void your home insurance.
Kite marks confirm building regulation compliance
Building regulations for England and Wales require replacement windows to meet specific energy efficiency standards. The BSI Kitemark for windows confirms the product has been independently tested to BS 6375 (weather tightness) and BS EN 14351-1 (product standard). The Energy Saving Trust states that certified windows typically reduce heat loss by up to 50% compared to single glazing (Energy Saving Trust, 2026). Without a kite mark, you have no independent guarantee that your windows achieve the required U-value of 1.6 W/m²K for new builds or 1.4 W/m²K for replacement glazing in existing homes.
Only FENSA installers can bypass the kite mark
If you use a FENSA-registered or CERTASS-registered installer, they are legally responsible for ensuring the windows meet building regulations. The installer provides a certificate of compliance, and a kite mark on the glass itself is not essential. However, the Glass and Glazing Federation advises that even with certified installers, windows with a kite mark offer stronger consumer protection and warranty cover (FENSA, 2026). For DIY installations or purchases from unregistered suppliers, a kite mark becomes the only reliable proof of compliance.
Kite marks protect your home insurance and resale value
Home insurance policies often require windows to meet building regulations. Without a kite mark or FENSA certificate, an insurer may refuse a claim for damage caused by poorly fitted or substandard glazing. The BSI Kitemark also simplifies future home sales, solicitors routinely ask for proof that replacement windows comply with regulations. The TrustMark scheme, endorsed by the UK government, recommends kite-marked products for all home improvements (TrustMark, 2026). A kite mark adds around 5–10% to the window cost but avoids expensive retrofitting and legal disputes.
A worked example
For a typical 1930s semi-detached house in Manchester replacing 10 standard double glazed windows, a BSI Kitemarked set costs around £4,800 fully installed by a FENSA-registered company, while an uncertified equivalent bought online and fitted by a local handyman might cost £3,200 but risks failing building control inspection. The Energy Saving Trust estimates that poorly sealed windows can increase heating bills by up to 15%, adding roughly £180 per year to gas bills for a gas-heated home.Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The uncertified windows, if they fail a thermal test, could require replacement at full cost within two years, wiping out any initial saving. A Kitemark from BSI or a UKAS-accredited body is the simplest way to guarantee a U-value of 1.6 W/m²K or better, as required by Approved Document L.
| Item | Figure |
|---|---|
| Upfront cost after grants | £4,800 |
| Yearly savings | £180 |
| Payback period | 27 years |
| 25-year lifetime savings | £4,500 |
What homeowners often get wrong
The most common mistake is assuming every double glazed window must display a visible kite mark to be legal. This misunderstanding leads to three specific errors that cost UK homeowners money and compliance headaches.
- Believing a kite mark is always mandatory Many homeowners reject perfectly compliant windows because they lack a visible BSI logo. The truth is that FENSA or CERTASS registered installers can self-certify compliance without a kite mark, and their certificate is legally sufficient for building control sign-off. Rejecting such windows can delay a project by weeks and add £300 in unnecessary reordering costs.
- Buying cheap online windows without any certification A DIY purchase from an online retailer with no kite mark and no registered installer is the riskiest route. Without a kite mark, you have no proof the glazing meets the 1.6 W/m²K U-value requirement, and a building control inspection could demand removal. That mistake can cost £4,000 in wasted windows plus £1,500 in enforcement fees.
- Ignoring the kite mark on secondary glazing Many homeowners think only primary double glazing needs certification. In fact, secondary glazing sold as a permanent thermal upgrade also requires a Kitemark or equivalent certification under building regulations. Installing uncertified secondary glazing can void your home insurance and fail an EPC assessment, reducing your property value by up to 5%.
Quick reference
- A BSI Kitemark guarantees a U-value of 1.6 W/m²K or better for new double glazed windows in England and Wales.
- FENSA or CERTASS registered installers can self-certify compliance without a visible kite mark on every window.
- DIY window installations without a kite mark risk failing building control inspection and costing £4,000 or more to replace.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
- Secondary glazing used as a permanent thermal upgrade also requires a kite mark or equivalent certification under building regulations.
Frequently Asked Questions
No, not all double glazed windows need a kite mark. If a FENSA or CERTASS registered installer fits them, they self-certify building regulation compliance, making a separate kite mark optional.
A kite mark, such as the BSI Kitemark, confirms the window has been independently tested to BS 6375 for weather tightness and BS EN 14351-1 for product standards. The Energy Saving Trust says certified windows can cut heat loss by up to 50% compared to single glazing.
Yes, you can fit double glazed windows without a kite mark, but you must self-certify compliance with building regulations via a local authority building control application. Without certification, your windows may fail thermal efficiency standards and could void home insurance.
