What U-Value Do I Need?
Quick Answer — Approved Document L 2021 (England)
Walls: 0.30 W/m²K
Roof: 0.16 W/m²K
Floor: 0.25 W/m²K
Windows: 1.4 W/m²K
Doors: 1.4 W/m²K
These are the maximum (worst) permitted U-values for new-build dwellings in England under Part L 2021
What Is a U-Value?
A U-value measures how much heat passes through a building element(wall, roof, floor, window). It is expressed in W/m²K (watts per square metre per degree Kelvin). The lower the U-value, the better the insulation.
New-Build U-Value Targets (Part L 2021, England)
The 2021 update to Approved Document L significantly tightened U-value requirements for new homes:
- External walls: 0.30 W/m²K (previously 0.30 under 2013 regs)
- Pitched roof (insulation at ceiling level): 0.16 W/m²K
- Pitched roof (insulation at rafter level): 0.16 W/m²K
- Flat roof: 0.18 W/m²K
- Ground floor: 0.25 W/m²K
- Windows, roof windows, curtain walling: 1.4 W/m²K
- External doors: 1.4 W/m²K
- Party walls: 0.20 W/m²K (between dwellings)
Extension and Renovation U-Values
Extensions and renovations also need to meet U-value targets. The 2021 requirements for extensions are the same as for new build. For renovation of existing thermal elements, the “threshold” trigger values and “improved” target values are:
- Wall: Threshold 0.70, Target 0.30 W/m²K
- Pitched roof (ceiling): Threshold 0.35, Target 0.16 W/m²K
- Flat roof: Threshold 0.35, Target 0.18 W/m²K
- Floor: Threshold 0.70, Target 0.25 W/m²K
If your existing element already performs better than the threshold, you do not need to upgrade when renovating. If it is worse than the threshold, you must improve it to the target value (or as close as reasonably practicable).
Scotland, Wales and Northern Ireland
Each nation has its own Building Regulations with slightly different U-value targets:
- Scotland (Section 6, 2022): Walls 0.22, Roof 0.15, Floor 0.18, Windows 1.4
- Wales (Part L 2022): Similar to England 2021
- Northern Ireland (Part F, 2022): Walls 0.30, Roof 0.16, Floor 0.25, Windows 1.4
Typical U-Values of Common Constructions
- Solid 9" brick wall (uninsulated): ~2.1 W/m²K
- Unfilled cavity wall: ~1.5 W/m²K
- Filled cavity wall (50mm): ~0.50 W/m²K
- Modern insulated cavity wall: ~0.25–0.30 W/m²K
- Loft with 100mm mineral wool: ~0.44 W/m²K
- Loft with 270mm mineral wool: ~0.16 W/m²K
- Single glazing: ~5.0 W/m²K
- Double glazing (modern): ~1.2–1.4 W/m²K
- Triple glazing: ~0.8 W/m²K
Tips
When submitting plans to Building Control, you will need to demonstrate compliance with Part L using either the elemental method (meeting each U-value individually) or the whole-building method (SAP calculation showing overall energy performance). The elemental targets above are the simplest route for extensions and straightforward new builds.
Always check the current edition of Approved Document L for your nation, as standards are updated periodically and the Future Homes Standard (expected 2025) will tighten requirements further.