Part L compliance
Part L limiting U-value check for renovation (existing dwellings)
- where
- existing U-value above the threshold (W/m²K)
- existing U-value of the element (W/m²K)
- threshold U-value (W/m²K)
- margin of the proposed U-value below the limit (W/m²K)
- limiting (improved) U-value (W/m²K)
- proposed U-value after work (W/m²K)
Explain this equation
Checks a renovation against the Approved Document L rules for existing dwellings: whether the element must be upgraded, and whether the proposed U-value meets the limit.
When you renovate a wall, roof or floor whose U-value is worse than the threshold (a positive Δ_t), Part L expects it to be upgraded to the limiting U-value, as long as that is technically, functionally and economically feasible. A positive Δ_l means your proposed U-value is at or better than the limit.
Two simple differences against the values in the Approved Document tables.
Sense check: a solid wall at 2.0 is 1.3 above the 0.70 threshold, so it should be upgraded; a proposed 0.28 is 0.02 better than the 0.30 limit for internal or external wall insulation.
Watch out: if meeting the limit is not feasible (for example payback over 15 years or excessive loss of floor area), aim for the best reasonable U-value and record why. Building control may need notifying.
Compares an existing and a proposed U-value with the Approved Document L figures for renovating thermal elements in existing dwellings.
Enter your values to see the result.
Good to know
Approved Document L Volume 1 (England), Table 3.3 of the 2026 edition and Table 4.3 of the 2021 edition (same values): threshold / improved U-values in W/m²K are roof 0.35 / 0.16; wall with cavity insulation 0.70 / 0.55; wall with internal or external insulation 0.70 / 0.30; floor 0.70 / 0.25. New or replacement elements in an existing dwelling (extensions, new walls) must meet roof 0.15, wall 0.18 and floor 0.18 (Table 3.2). The 2026 edition comes into force on 24 March 2027 for most work; until then the 2021 edition applies, subject to the transitional arrangements. Renovation of more than 50% of an element's surface, or re-rendering, re-plastering or re-roofing, triggers the requirement. Wales and Scotland have their own documents and values.
Common mistake
Applying the new-build or extension values (0.18 walls) to the upgrade of an existing solid wall, or ignoring the requirement entirely when re-plastering or re-rendering more than half a wall.
Limits
Element-by-element check for England. Does not cover the alternative whole-dwelling approach, conservation-area or listed-building exemptions, or the Part C moisture check that must accompany an upgrade. Always confirm with building control.
Sources
- Approved Document L Volume 1: Dwellings, 2026 edition (England): Table 3.2 new or replacement elements in existing dwellings (roof 0.15, wall 0.18, floor 0.18 W/m²K); Table 3.3 existing elements: threshold / improved U-values roof 0.35/0.16, cavity fill 0.70/0.55, internal or external wall insulation 0.70/0.30, floor 0.70/0.25; para 3.14 15-year payback and 0.7 backstop. In force 24 March 2027; the same values appear in Tables 4.2 and 4.3 of the 2021 edition, which applies until then
- GOV.UK: Conservation of fuel and power – Approved Document L (current and 2021 editions, transitional arrangements)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.