Domestic U-values for Building Regulations by work category
The selected figures below come from Tables 4.1 and 4.2 of Approved Document L, Volume 1, 2021 edition incorporating 2023 amendments. They are maximum U-values in W/(m²·K), subject to the tables' notes and the surrounding guidance. They cover the listed domestic categories only, not every construction or exception.
Our U-value calculations service can assess a proposed assembly once its layers and work category are known. Explain whether you are constructing a new home, adding an extension, replacing an element or improving an existing assembly. Those descriptions affect which guidance should be consulted.
| Element | New dwelling: Table 4.1 | New element in existing dwelling: Table 4.2 |
|---|---|---|
| Roof | 0.16 W/(m²·K) | 0.15 W/(m²·K) |
| Wall | 0.26 W/(m²·K) | 0.18 W/(m²·K) |
| Floor | 0.18 W/(m²·K) | 0.18 W/(m²·K) |
Source: Approved Document L, Volume 1, 2021 edition incorporating 2023 amendments, Table 4.1 (new fabric elements in new dwellings: roof 0.16, wall 0.26, floor 0.18) and Table 4.2 (new fabric elements in existing dwellings: roof 0.15, wall 0.18, floor 0.18). Values in W/(m²·K), subject to the notes under each table.
Why Building Regulations U-values are not a complete specification
The new-dwelling table sets limiting fabric standards, but Approved Document L, Volume 1, 2021 edition incorporating 2023 amendments warns that some elements need significantly better performance to meet the target fabric energy efficiency rate. A design that uses every limiting value is therefore not automatically a compliant whole-home design.
Keep the fabric schedule coordinated with the energy assessor's model. If the model relies on a better wall than the minimum table entry, that better value belongs in the project specification. Substituting the table maximum without checking the assessment can remove performance that the designer was relying on elsewhere.
For extensions, the relationship between glazing and fabric can affect the assessment route. Our SAP calculations for extensions guide discusses that choice. For a new dwelling, the new-build compliance package helps place fabric calculations alongside the other evidence being coordinated for the project.
Existing fabric and non-domestic Building Regulations U-values
Renovating an existing wall or roof is a different situation from forming a new extension wall. Approved Document L, Volume 1 includes separate renovation guidance and considerations of feasibility. Describe what is being retained, removed and replaced so that the designer can identify the relevant provisions instead of taking the nearest-looking table entry.
Buildings other than dwellings use Approved Document L, Volume 2. Do not carry the domestic figures above into an office or warehouse specification. Where a site combines flats and commercial space, mark the use boundaries and discuss them with the assessor; our SAP vs SBEM guide explains the assessment distinction.
For awkward existing details, record the constraint rather than silently reducing insulation. Floor levels, roof geometry and adjoining construction can affect practical options. Ask the designer to document the proposed response and its assessment basis before procurement fixes the build-up. A general note saying insulation to comply gives the contractor little useful information.
What the 2026 Approved Document L says about U-values
Approved Document L, Volume 1, 2026 edition states that it takes effect on 24 March 2027 for work not connected with higher-risk building work and on 24 September 2027 for work connected with higher-risk building work. It directs readers to the transitional arrangements. Publication of a newer edition does not by itself identify the edition applicable to a particular project.
The selected roof, wall and floor values shown above are unchanged in Tables 3.1 and 3.2 of Approved Document L, Volume 1, 2026 edition. This does not mean that every part of the energy assessment remains unchanged. Confirm the applicable edition and project circumstances before relying on a reused specification.
Evidence behind U-values for Building Regulations
A common reason a U-value is queried is that the calculation describes a different build-up from the one on site. Insulation is swapped for a product with another thickness or conductivity, a stud spacing changes, or a membrane is added late. Each of these can move the result. When the specification changes, ask for the calculation to be rerun before the element is built, so the figure Building Control sees matches what was installed.
Give the calculator an assembly description, not just an insulation brochure. Include internal and external layers, framing information, cavities and fixings, with product data that identifies the intended materials. For a ground floor, supply the geometry. Mark each build-up on the drawings so that a reviewer can connect the result to its location.
A supplier substitution needs more than confirmation that the new board has the same thickness. Check its declared properties and whether the assembly details have changed. Send the revised information through our U-value calculations service with the original report reference, affected drawing and proposed replacement, so that the assessed construction remains identifiable.