ProjectCalcWhich edition applies to your job
Part L is the one Approved Document where the edition depends on the regulatory standard the work is being carried out under. GOV.UK publishes two current documents and its own wording is that "Approved Document L (2026) provides guidance on meeting the energy efficiency requirements of Part L of the Building Regulations for buildings subject to the 2026 standards", and that "earlier versions of Approved Document L continue to apply to buildings subject to previous regulatory standards."
So there are two live documents: the 2021 edition incorporating 2023 amendments (Volume 1: Dwellings) and the 2026 edition (Volume 1: Dwellings, titled Energy and greenhouse gas emissions). Which one governs your work is a question for your building control body.
The limiting values themselves are the same in both editions, which is why the table below is safe to publish with that caveat attached.
Limiting U-values for new fabric elements in new dwellings
| Element | Maximum U-value W/(m²·K) |
|---|---|
| All roof types | 0.16 |
| Wall | 0.26 |
| Floor | 0.18 |
| Party wall | 0.20 |
| Swimming pool basin | 0.25 |
| Window | 1.6 |
| Rooflight | 2.2 |
| Doors (including glazed doors) | 1.6 |
| Air permeability at 50 Pa | 8.0 m³/(h·m²) |
Two notes from the table matter. The values are area-weighted average values, and for dormer windows the "roof" row includes the roof parts of the windows while the "wall" row includes the wall parts (the cheeks).
The document is also careful to say that these are limits, not targets: to meet the target fabric energy efficiency rate set out in Section 1, the energy efficiency of some elements will need to be significantly greater than the limiting standards.
Limiting values for new elements in existing dwellings
This is the table that applies to the common domestic job — an extension, a replacement window, a new floor. Table 3.2 (2026 edition) / Table 4.2 (2021 edition) is stricter on walls and windows than the new-build table:
| Element | Maximum U-value |
|---|---|
| Roof | 0.15 W/(m²·K) |
| Wall | 0.18 W/(m²·K) |
| Floor | 0.18 W/(m²·K) |
| Window | 1.4 W/(m²·K) or Window Energy Rating Band B minimum |
| Doors with more than 60% of the internal face glazed | 1.4 W/(m²·K) or Doorset Energy Rating Band C minimum |
| Other doors | 1.4 W/(m²·K) or Doorset Energy Rating Band B minimum |
The document states that elements which should meet these standards include elements in extensions to existing dwellings, and new or replacement elements in existing dwellings. Where an element is being replaced, the U-value should both be no higher than that of the element being replaced and meet the limiting standard.
Two of the footnotes are practical relief valves. If meeting the wall standard would reduce the internal floor area of the room bounded by that wall by more than 5%, a lesser provision may be appropriate. And if meeting the floor standard would create significant problems in relation to adjoining floor levels, a lesser provision may be appropriate. Both are allowances within the document, not general permission — they still go through building control.
How U-values are assessed, and what that means for a specification
U-values are assessed using the methods and conventions in BRE's BR 443, for the whole fabric element — not for a sample of it. For a window or door in a new dwelling, the U-value is determined to include the frame and the glazing and the effect of the spacer, which is why a whole-unit figure is worth more than a centre-pane figure when you are comparing products.
For a job, the practical consequence is straightforward: an insulation product's own declared performance is not the same as the U-value of the element as built. The build-up, the fixings and the thermal bridges all count, and that is what an energy calculation is for.
Last verified 22 September 2026. Every requirement quoted on this page was read from the primary source listed below on that date, not from a summary. If a figure could not be confirmed in the primary source it is not on this page.
Where these figures come from
- Approved Document L, Volume 1: Dwellings — 2026 edition, "Energy and greenhouse gas emissions" (Table 3.1; Section 3) — GOV.UK publications, "Conservation of fuel and power: Approved Document L", checked 22 September 2026.
- Approved Document L, Conservation of fuel and power, Volume 1: Dwellings — 2021 edition incorporating 2023 amendments (Tables 4.1 and 4.2) — GOV.UK publications, "Conservation of fuel and power: Approved Document L", checked 22 September 2026.
Guidance only, not advice
This page is a plain-English summary of published regulations and standards, written to help you plan a job. It is not professional advice, it is not a substitute for the actual document, and it is not a building-control decision.
Building regulations are enforced by your local authority (or an approved inspector) and the facts of a particular job can change the answer. Confirm with your local building control before you start work, and where a job is structural, get it designed by a suitably qualified person. If this page and the source document disagree, the source document wins.