What is the difference between a target U-value and a limiting U-value?

Target U-values (also called notional dwelling values) are the values used in the notional dwelling SAP calculation and represent the expected performance level. Limiting U-values are the absolute worst-case values that individual elements must not exceed, regardless of how well other elements perform. You cannot compensate for a poor wall U-value by having better windows, for example - every element must at least meet the limiting value.

Do extensions have different U-value targets?

Yes. Extensions and renovations have slightly less stringent U-value requirements than new-build dwellings. For example, extension walls must achieve 0.18 W/m²K (same as new build in practice), windows 1.4 W/m²K (vs 1.2 for new build), and flat roofs 0.18 W/m²K. These values are set out in Table 4 of Approved Document L Vol 1 (extensions) and Table 5 (renovations).

What changed from Part L 2013 to Part L 2021?

Part L 2021 tightened several U-value targets: floor U-values dropped from 0.13 to 0.13 W/m²K (unchanged), wall U-values from 0.18 to 0.18 W/m²K (unchanged), but window targets dropped from 1.4 to 1.2 W/m²K. The biggest changes were to the carbon emission factors and the introduction of the Primary Energy Rate (TPER) as an additional compliance metric alongside carbon emissions.

How do I achieve a wall U-value of 0.18 W/m²K?

A typical cavity wall construction to achieve 0.18 W/m²K would use: brick outer leaf, 150 mm full-fill mineral wool cavity insulation (lambda 0.032-0.034), 100 mm aerated concrete block inner leaf (lambda 0.11), 12.5 mm plasterboard on dabs. Alternative constructions include partial fill PIR insulation (100 mm with a 50 mm residual cavity) or timber frame with 140 mm mineral wool between studs plus a service void.

Do these U-values apply in Scotland, Wales and Northern Ireland?

No. These are specifically for England under Approved Document L 2021. Scotland has Section 6 of the Building Standards (generally stricter), Wales has its own Part L (broadly similar but with some differences), and Northern Ireland has Part F (Conservation of Fuel and Power). Always check the specific regulations for the country where you are building.

Part L U-Values Reference — Free Trade Calculator

Part L 2021 U-Value Targets — All Building Elements

Complete Part L 2021 U-value targets for new builds and extensions in England, including walls, floors, roofs, windows, and doors.

New-Build Dwellings (Part L Vol 1, 2021)

Element	Notional Target (W/m²K)	Limiting Value (W/m²K)
External wall	0.18	0.26
Party wall (between dwellings)	0.00	-
Floor (ground floor)	0.13	0.18
Pitched roof (insulation at ceiling)	0.11	0.16
Pitched roof (insulation at rafter)	0.11	0.18
Flat roof	0.11	0.18
Windows	1.2	1.6
Roof windows	1.2	1.6
External doors (opaque)	1.0	1.6
Semi-glazed doors (>60% glazed)	1.2	1.6

Extensions to Existing Dwellings

Element	Target U-value (W/m²K)
External wall	0.18
Floor	0.18
Pitched roof (at ceiling)	0.15
Flat roof / pitched roof (at rafter)	0.18
Windows, roof windows, glazed doors	1.4
Opaque doors	1.4

Renovation of Existing Elements

Element	Target U-value (W/m²K)
Wall (cavity insulation)	0.55
Wall (internal/external insulation)	0.30
Floor	0.25
Pitched roof (at ceiling)	0.16
Flat roof or pitched roof (at rafter)	0.18
Windows, doors	1.4

Related Calculators

Use the U-Value Calculator to check your construction meets these targets, or the Insulation Thickness Calculator to find the required insulation depth. See also the SAP glossary entry and Thermal Bridge glossary entry.

How We Calculate This

Frequently Asked Questions

Last updated: March 2026

All calculations are estimates. Verify with your supplier.

Part L 2021 U-Value Targets — All Building Elements

New-Build Dwellings (Part L Vol 1, 2021)

Extensions to Existing Dwellings

Renovation of Existing Elements

Related Calculators

How We Calculate This

Frequently Asked Questions

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