England’s next Part L standards are now published, giving developers and design teams a defined timetable for the move to the 2026 edition of Approved Document L and the updated SAP 10.3 calculation methodology.
The new Approved Document L was published on 24 March 2026. For most building work in England it takes effect on 24 March 2027, with separate timing for higher-risk building work from 24 September 2027. The change matters now because projects entering design and planning during 2026 may be constructed under the new standards depending on when notices, plans and work commence.
SAP 10.3 is the approved domestic calculation methodology
The government’s March 2026 notice of approval updated the Standard Assessment Procedure from SAP 10.2 to SAP 10.3 for new dwellings in England. SAP is used to calculate the energy performance metrics required to demonstrate compliance with the Building Regulations, including target and calculated carbon emissions, fabric energy efficiency and primary energy rates.
This does not mean every current project immediately moves to the new Approved Document L. Transitional arrangements are important, and the applicable standard depends on the regulatory route and project dates.
When does Approved Document L 2026 take effect?
For building work that is not connected with a higher-risk building, Approved Document L 2026 takes effect on 24 March 2027. The approved document states a later effective date of 24 September 2027 for work connected with higher-risk buildings.
The government’s methodology notice also provides transitional protection in certain circumstances. For non-higher-risk buildings, earlier calculation methodologies can continue where the relevant building notice, initial notice or full plans were submitted before 24 March 2027 and the building work starts before 24 March 2028. Higher-risk buildings have separate Gateway 2 transitional arrangements.
Because transition depends on the individual project route, developers should not assume that a scheme designed during 2026 will automatically be assessed under the same standard when construction begins.
Why early SAP modelling becomes more important
A SAP assessment is most useful when it informs design rather than simply checking a completed specification. Fabric performance, glazing, thermal bridging, heating systems, ventilation, renewables and other design decisions interact within the calculation.
If a project is likely to fall under the 2026 standards, modelling the design early can identify compliance pressure before specifications are fixed. That gives the design team more scope to balance fabric, services and renewable technologies without late redesign.
Air tightness and the performance gap
Part L compliance is not just a desktop exercise. The as-built performance of the building matters, and air tightness testing provides evidence of how the completed envelope performs against the design assumptions.
Where the tested air permeability differs from the value assumed in the energy model, the as-built SAP result can also change. Coordinating SAP calculations, construction detailing and air tightness testing therefore becomes increasingly important as energy standards tighten.
What about the Home Energy Model?
The government continues to develop the Home Energy Model as the longer-term methodology for new dwellings. The 2026 Future Homes and Buildings Standards consultation response confirmed SAP 10.3 as an approved route while the Home Energy Model is introduced, with a planned dual-running period before any eventual move away from SAP.
For current projects, the practical message is simple: SAP remains directly relevant, but project teams need to use the version and regulatory standard that applies to their specific transition date.
EnviroSolution supports residential developments with SAP calculations, energy compliance advice and air tightness testing. If you are planning a project that may cross the March 2027 transition, we can review the programme and design information before the compliance route is fixed.