UK Building Regs Part L: Fenster & Fassaden (GB)

The 2021 edition of Approved Document Part L (Conservation of Fuel and Power), which became mandatory in England from 15 June 2022, introduced significantly tighter thermal performance requirements for the building envelope — including windows, doors, rooflights and curtain walling. This update forms an interim step towards the Future Homes Standard, which is expected to require near-zero-carbon new homes by 2025/2026. For the fenestration, door and façade industry, these changes represent both a compliance obligation and a commercial opportunity to supply higher-performing products.

Part L applies to all new dwellings (Volume 1) and new non-domestic buildings (Volume 2), as well as extensions, material alterations and changes of use in existing buildings. Housebuilders, developers, main contractors and self-builders in England must demonstrate compliance via an approved calculation methodology — typically SAP 10.2 for dwellings or SBEM for commercial buildings. NHBC, Local Authority Building Control (LABC) and approved inspectors are the primary enforcement bodies. Note that Scotland, Wales and Northern Ireland operate under separate but broadly similar devolved regulations.

Under Part L 2021, replacement windows in new-build dwellings must achieve a whole-window U-value of no more than 1.2 W/m²K, while doors (with more than 60% glazing) must meet 1.4 W/m²K. For non-domestic façades, limiting U-values and the trade-off methodology via a building energy model must be applied. Solar gain (g-value) must also be controlled to avoid overheating, which is now assessed under the linked Approved Document O (Overheating). Products should carry CE/UKCA marking and third-party certification (e.g., BFRC, BSI Kitemark) to demonstrate conformity efficiently during building control inspections.

For fenestration contractors and façade suppliers, the practical compliance route involves: ensuring all specified products carry valid UKCA/CE performance data (U-value, air permeability, weather resistance); providing test evidence per EN 14351-1 for windows and doors; working with the SAP/SBEM assessor from design stage; and retaining on-site documentation for the building control sign-off. With the Future Homes Standard consultation ongoing, businesses should also track proposed further tightening of U-value targets (potentially 0.8–1.0 W/m²K for windows) and plan product development accordingly.

Note: This article is not a substitute for legal or financial advice. Please consult a qualified expert for guidance specific to your circumstances.

Umsetzungs-Checkliste

1. Confirm which Part L volume applies (Volume 1 dwellings / Volume 2 non-domestic)
2. Specify windows and doors with whole-unit U-values ≤1.2 W/m²K (windows) and ≤1.4 W/m²K (doors)
3. Ensure all products carry valid UKCA or CE marking and EN 14351-1 test evidence
4. Engage a SAP 10.2 (dwellings) or SBEM (commercial) assessor at design stage
5. Check solar gain (g-value) compliance against Approved Document O overheating criteria
6. Prepare and retain product performance documentation for building control inspection
7. Obtain NHBC or LABC sign-off before practical completion
8. Monitor Future Homes Standard consultations for upcoming U-value tightening post-2026