Consultation outcome confirms Welsh Building Regulations changes from March 2027
On 27th February 2026 The Welsh government published the responses to a consultation on changes to the next versions of Part F, L and O of Welsh Building Regulations for both dwellings and non-domestic buildings. The consultation, Building Regulations Part L: 2025 review, proposed numerous changes to the compliance standards and the associated calculation methodologies, particularly looking at the introduction of the Home Energy Model for new build dwellings. This article explores some of the changes that have been proposed.
New Domestic Dwellings
Uplift to Energy Efficiency Standards
The consultation proposed that new homes be designed to achieve net zero carbon emissions as the electricity grid decarbonises. Two uplift options over Part L 2022 were outlined, both assuming a heat pump and solar PV in the notional dwelling:
- Option 1: DMEV ventilation with an airtightness test score of 4 m3/m2/hr @ 50 Pa.
- Option 2: MVHR ventilation with an airtightness test score of 1.5 m3/m2/hr @ 50 Pa
Government Outcome: The Government have confirmed that option 1 which delivers a 93% carbon saving over Part L 2022 at a build cost increase of 3.3% will be chosen to form the basis of the notional dwelling responsible for setting the performance standards in Part L 2025. However there were a couple of changes to the notional dwelling proposed in the consultation;
- External wall u-value changed from 0.15 to 0.13
- Renewable energy generation will be set in the notional dwelling to the same level as the new functional requirement rather than 40% ground floor area/4.5.
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 proposed 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.
Government Outcome: The consultation responses confirmed the Dwelling Energy Efficiency Rate (DEER) will be replaced with Energy Use Intensity (EUI) in Part L 2025.
Calculation Methodology
In regards to the national calculation methodology used to demonstrate compliance with the energy efficiency standards in Part L 2025 it was proposed to use the Home Energy Model (HEM), with a ‘Wales Part L 2025’ wrapper as a replacement to the current Standard Assessment Procedure (SAP).
However, the consultation did leave an opportunity for SAP to continue if HEM is not completed in time for Part L 2025.
Government Outcome: Following the decision by MHCLG to delay the introduction of HEM for at least three months after the Future Homes Standard is published in England, the Welsh Government have also confirmed that SAP 10.3 will initially be used to calculate compliance to Part L 2025 with HEM coming later when it is deemed ready.
A requirement for renewable energy generation
Government Outcome: The responses confirmed a new functional requirement will be implemented into schedule 1 of the Building Regulations will be introduced stipulating a minimum amount of renewable energy generation on new homes. Additional guidance will be contained in the Approved Document in regards to scenarios where complex roof designs may limit the installation of sufficient photovoltaic panels to a point where the minimum requirement cannot be met.
Changes for Existing Dwellings
It was proposed for new ‘significant’ extensions to existing properties with over 30 m2 total internal floor area renewable energy must be installed during construction. This could be solar photovoltaic panels or solar hot water panels although there was some flexibility proposed which could result in heat pumps or other technologies being used instead.
Government Outcome: The consultation responses have seen some changes to this proposal. The definition of a significant extension has changed to an extension having a total internal floor area of at least 40 m2 and have at least as many stories as the existing dwelling. Some exemptions will also be applied to this requirement for example dwellings in conservation areas.
Changes to Part F – Ventilation
Of course changes to Part L specifically in the areas of airtightness, need to be reflected in Part F. Changes proposed for work to Ventilation systems included;
- 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.
Government Outcome: The consultation response did not give much information on the above but did confirm Competent Persons Schemes are currently under review with the Building Safety Regulator.
Changes to Part O – Overheating
Government Outcome: The consultation responses confirmed proposed changes to Part O will be published separately later this year.
New & Existing Non-Domestic Buildings
Uplift to Energy Efficiency Standards
Similarly to the proposed changes for new build dwellings, the consultation emphasised 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.
Government Outcome: the Welsh Government confirmed that it will proceed with Option 2, with adjustments to the notional PV coverage assumptions used in the NCM methodology. PV coverage will reduce from 50% to 40% for building types 1 & 2, and from 75% to 60% for building type 3, while all other Option 2 parameters remain unchanged.
Photographic Evidence
The consultation also proposed 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.
Government Outcome: With 92% of respondents supporting the above proposal, and no responses opposing it, the Welsh Government confirmed that the requirement will be included within the final Approved Document guidance.
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.
Government Outcome: The Welsh Government confirmed it will adopt the proposed improvements to minimum efficiencies for building services and controls, including heating, cooling and lighting systems. However, the proposed lighting efficacy requirements will be revised following stakeholder feedback.
Updates to Non-Domestic Calculation Methodology
The Welsh Government’s Part L 2025 review proposes updates to the National Calculation Method (NCM), which is used to assess the energy performance of non-domestic buildings. Draft versions of the updated NCM and the consultation version of SBEM (cSBEM) were available for stakeholders to review and provide feedback. To view the government cSBEM tool, please click here >
Government Outcome: The Government confirmed that, based on the consultation feedback, they will take these responses into account when finalising the proposed methodology in the NCM modelling guide for the Part L 2025 standard. These updates will underpin how compliance is demonstrated for non-domestic buildings, meaning assessors and modellers will need to familiarise themselves with the updated methodology ahead of implementation.
Timelines and Transitional Arrangements
The Welsh Government sent a circular to Building Control Bodies on 6th March 2025, found here, confirming that the new regulations will come into force on 4th March 2027. Following this there will be a 12 month transitional period as per previous changes to Part L of the Building Regulations.
From 4th March 2027 any homes subject to Building Regulations will need to be constructed to Part L 2025. For homes that are already under construction they will be able to be completed to the version of Part L that was in place when construction commenced.
For dwellings currently benefiting from sites under older transitional arrangements these will be ended. Known as ‘sunsetting older transitional arrangements’ for dwellings currently covered by older transitional arrangements, where work has not commenced on plots within the transition period, they will have to be constructed to Part L 2025. The final version of the approved documents will be made available in spring/summer 2026.
Jason Hewins, New Build Dwellings Scheme Manager comments:
“Elmhurst are pleased to see the Wales Government publish the consultation responses on the next revisions to Part F and L of the Building Regulations
We are pleased to see the Wales Government take the pragmatic step of allowing SAP 10.3 to be used to demonstrate compliance until the Home Energy Model is ready to take over. This will allow assessors to assess compliance for their clients efficiently using applications that are familiar and supported by clear conventions and guidance. Elmhurst will continue to support the Welsh Government with the implementation of the Home Energy Model to ensure its roll out is as smooth as possible for the industry.
The proposals are broadly as expected and we look forward to the publication of the Approved Documents and further details on Part O changes later this year.”
John Robinson, Non-Domestic Scheme Manger comments:
The Welsh Government’s response provides greater clarity on the direction of travel for non-domestic buildings under Part L. The confirmation that Option 2 will be implemented, alongside updates to the National Calculation Method, reinforces the need for robust modelling and compliance processes.
For energy assessors and building professionals, the key priority now will be preparing for the updated NCM methodology and ensuring that designs are capable of meeting these more ambitious performance targets.
Welsh Building Regulations Part L: 2025 review
Read the full government outcome below.
READ THE GOVERNMENT OUTCOME