PART L · Energy performance
Part L building regulations explained
Part L deals with energy performance. It affects insulation, windows, airtightness and fixed building services. The applicable requirements depend on whether the project is a new building, an extension, a conversion or other work to an existing building.
Is insulation the only issue
No. The fabric and services work together. Good insulation can be undermined by gaps, poorly detailed junctions or badly commissioned systems.
For a new dwelling, the compliance approach assesses more than a list of individual products. Relevant calculations and completion evidence need to describe the building that is actually constructed.
What rules apply in October 2026
The 2021 Approved Document L volumes incorporating 2023 amendments remain the main current guidance, subject to project-specific transitional arrangements. The newer 2026 energy and greenhouse gas emissions guidance has been published for forthcoming standards.
The associated regulations generally commence on 24 March 2027. Specified higher-risk building provisions commence later, on 24 September 2027. Publication alone does not make the new regime apply to every project today.
What should an extension owner ask
Ask how walls, floors, roofs and openings meet the applicable approach. If substantial glazing is proposed, establish whether additional calculations are needed before the windows are ordered.
Make sure the heating and ventilation plans are coordinated. Part F fresh-air provision should not be defeated by attempts to reduce heat loss.
What evidence matters
Keep the specification, relevant calculations, installation records and commissioning information. For work requiring testing or other compliance evidence, agree the provider and timing early.
Review substitutions. A different window, insulation product or heating arrangement can change the assessment.
Record the applicable edition and transition basis in the project file. The newest guidance is useful for future design, but the correct legal standard depends on the particular project and commencement provisions.