Building Regulations U-Values for Walls, Roofs and Floors (Part L 2021)

For a new-build dwelling in England, Approved Document L 2021 sets a limiting (backstop) U-value of 0.26 W/m²K for external walls and 0.16 W/m²K for roofs, with tighter notional design values of 0.18 W/m²K (wall) and 0.11 W/m²K (roof) used in the SAP calculation. For an extension, new walls and roofs must meet Table 4.2: 0.18 W/m²K (wall) and 0.15 W/m²K (roof). Renovating a retained wall or roof uses Table 4.3: wall 0.55 W/m²K by cavity fill or 0.30 W/m²K by internal or external insulation, roof 0.16 W/m²K. See the full tables below for floors, windows and doors.

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.

Yes, but they are different rather than simply looser. New elements in an extension must meet AD L 2021 (as amended) Table 4.2: walls 0.18 W/m²K, floors 0.18, roofs 0.15 (a single value covering pitched and flat), windows 1.4 (or Window Energy Rating Band B), doors 1.4 and rooflights 2.2. The wall, floor and roof limits are stricter than the new-build limiting values because an extension has no whole-dwelling SAP trade-off. Upgrading a retained existing element uses Table 4.3: wall 0.55 by cavity fill or 0.30 by internal or external insulation, floor 0.25, roof 0.16. Table 4.3 has no windows row: replacement windows fall under Table 4.2 at 1.4 W/m²K.

Part L 2021 tightened U-value targets compared to Part L 2013. Key changes: wall limiting dropped from 0.30 to 0.26 W/m²K, pitched roof limiting from 0.20 to 0.16 W/m²K, window targets from 1.4 to 1.2 W/m²K (new-build notional). The notional dwelling target (SAP compliance benchmark) uses 0.18 for walls, 0.11 for roofs and 0.13 for floors; the roof value tightened from 0.13 in 2013, while the wall and floor notional values carried over. 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.

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.

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.