Welsh Building Regulations Update 2026
Welsh Building Regulations Update 2026 (Parts L, F and O)
The Welsh Government published its consultation responses on 27 February 2026, confirming major updates to Building Regulations Parts L (Energy), F (Ventilation) and O (Overheating).
The changes introduce a significant uplift in energy-efficiency standards, a new performance metric (EUI), mandatory renewable-energy generation for new dwellings, and strengthened commissioning requirements.
The Government selected Option 1 as the basis for the new notional dwelling, aligning with viability evidence and the Future Homes Standard. SAP 10.3 will be used initially, with the Home Energy Model (HEM) introduced later once validated.
Regulations come into force on 4 March 2027, with a 12 month transitional period and the sunsetting of older transitional arrangements.
1.1 Part L – Energy Efficiency (New Dwellings): Uplift to Standards
The Government has adopted Option 1, defined as:
- DMEV ventilation
- Airtightness: 4 m³/m²/hr @ 50 Pa
- Heat pump
- Solar PV in the notional dwelling
- Supporting evidence notes this option provides a 93% carbon saving over Part L 2022 with an estimated 3.3% build cost increase.
- Two refinements to the notional dwelling were confirmed:
- External wall Uvalue tightened from 0.15 to 0.13 W/m²K
- Renewable generation in the notional dwelling will match the new functional requirement.
Welsh Building Regulations Update 2026 (Parts L, F and O)
Minimum Renewable Energy Requirement
- A new functional requirement mandates a minimum onsite renewable energy generation (typically PV).
- Guidance will address constrained roof scenarios and acceptable alternative evidence.
Performance Metric – EUI
- DEER will be replaced with Energy Use Intensity (EUI) for Part L 2025.
- EUI includes regulated and unregulated energy use and accounts for onsite renewable utilisation.
Calculation Method – SAP vs HEM
- SAP 10.3 will be used initially for compliance.
- HEM will be adopted later once validated and aligned with the Future Homes Standard rollout.
1.2 Part F – Ventilation
Changes to airtightness and energy standards drive consequential updates to Part F.
Key points:
- Ventilation systems must be commissioned by a Competent Person Scheme member or Building Control Body.
- Powered flow hoods will be required for commissioning; rotating vane anemometers will no longer be permitted.
- Competent Person Schemes are under review with the Building Safety Regulator
- MVHR will not be mandated in the notional dwelling due to skills shortages and airtightness concerns.
Welsh Building Regulations Update 2026 (Parts L, F and O)
1.3 Part O – Overheating
Updated guidance will be published later in 2026.
2. Timelines & Transitional Arrangements
Regulations come into force on 4 March 2027.
12 month transitional period applies.
Older transitional arrangements will be sunset.
“Sunsetting older transitional arrangements… where work has not commenced… they will have to be constructed to Part L 2025.”
Final Approved Documents expected spring/summer 2026.
Note: SAP 10.3 and HEM haven’t been released yet, so we’re currently unable to model anything. SAP 10.3 is expected to be released this quarter for England and will hopefully incorporate the Welsh requirements, allowing us to model Welsh SAP’s as well.