The Welsh government have launched a consultation looking at the Buildings Regulations Part L: 2025 Review for both dwellings and non-domestic buildings. The consultation looks at proposed changes and the associated calculation methodologies, particularly looking at the introduction of the Home Energy Model for new build dwellings. This consultation closes on 17^th November 2025.

Consultation Summary

Elmhurst take a look at some of the key changes proposed within this consultation:

New & Existing Dwellings

Uplift to Energy Efficiency Standards – The consultation proposes that new homes be designed to achieve zero carbon emissions as the electricity grid decarbonises. Two uplift options to Part L are outlined, both assuming a heat pump and solar PV in the notional dwelling:

• Option 1: DMEV ventilation.
• Option 2: MVHR ventilation – higher carbon and cost savings, but with greater build costs.

Performance Metrics – Part L 2022 currently uses the Dwelling Energy Efficiency Rate (DEER) which is a minimum EPC rating of a B for new homes. The consultation proposes to replace this with Energy Use Intensity (EUI) which is the total energy consumption from regulated and non-regulated (appliances) usage and inclusive of renewable energy generation that is used within the dwelling.

Other Part L changes include:

• Proposal for a functional requirement for a minimum amount of renewable energy generation on new homes.
• Using a single weather location of Cardiff rather than the current weather location of West Pennines, allowing for more accurate sizing of heat pumps.
• Either a 6 or 12 month implementation period. This is the timeframe between publication and commencement of Building Regulations.
• Sunsetting older transitional arrangements. Work that has not commenced on sites benefitting from older transitional arrangements will have to be constructed to Part L 2025.

Changes to Part F – Ventilation

The following changes are proposed for Part F:

• Clarifying that new ventilation systems installed in new and existing homes must be commissioned by a member of a competent person’s scheme or by the Building Control Body.
• Powered flow hoods must be used for commissioning new ventilation systems and rotating vane anemometers will no longer be permitted

Changes for Existing Dwellings – For significant new extensions to existing properties (over 30 m² total floor area) renewable energy must be installed during construction. This can be solar photovoltaic panels or solar hot water panels. In addition, two high-risk scenarios will be brought under the new Part O:

• Replacing windows in highly glazed flats
• Loft conversions with new openings

In both cases, solar gains must be controlled through appropriate mitigation measures.

New & Existing Non-Domestic Buildings

• Uplift to Energy Efficiency Standards – Similarly to the proposed changes for new build dwellings, the consultation emphasises the importance of integrating low-carbon heating systems, such as heat pumps or renewable electric systems, directly into the building specification. The goal is to ensure that these buildings are able to adapt to the decarbonisation of the electricity grid without requiring costly retrofit work later.
• Photographic Evidence – The consultation also proposes requiring photographic evidence for new non-domestic buildings. This would supplement current modelling tools and site inspections, providing a permanent record that key elements like insulation and building services are correctly installed.
• Improving Energy Efficiency for existing Non-Domestic Buildings – For existing non-domestic buildings, the consultation acknowledges the need to improve energy efficiency standards, though there are no specific proposals. The Welsh Government is seeking stakeholder input on practical measures that could be implemented to enhance performance in existing stock. This could include mandatory improvements during renovations or extensions.

Elmhurst Thoughts

Jason Hewins, New Build Dwellings Scheme Manager comments:

“Elmhurst are pleased to see the Wales Government publish this consultation on the next revision to Part F, L and O of the Building Regulations. We support the intent to revise all approved documents at the same time as they all must be considered together to prevent any unintended consequences.

We are pleased to see the Wales Government also state the intent to use the Home Energy Model if ready, but also take the pragmatic step of allowing SAP to be used if HEM is not at the position to be the sole methodology used to demonstrate compliance.”

John Robinson, Non-Domestic Scheme Manager goes on to add:

“Overall, the Part L 2025 review represents a significant step toward modernising energy performance standards for non-domestic buildings in Wales. By focusing on low-carbon heating, future-proofing new constructions, and exploring ways to enhance existing stock, the consultation reflects the government’s commitment to sustainability and the reduction of carbon emissions across the built environment.”

Further Information

https://www.gov.wales/building-regulations-part-l-2025-review

Home Energy Model: Part L Wales consultation tool: https://walespartl.consultationtool.org/

cSBEM: www.uk-ncm.org.uk