What U-Value Do I Need?
Quick Answer — Approved Document L 2021 (England) New-Build Limiting Values
Walls: 0.26 W/m²K
Roof (all types): 0.16 W/m²K
Floor: 0.18 W/m²K
Windows: 1.6 W/m²K limiting (1.2 notional)
Doors: 1.6 W/m²K limiting (1.0 notional)
These are the maximum (worst) permitted U-values for new-build dwellings in England under Part L 2021. Extension elements are stricter: walls 0.18, floors 0.18, roofs 0.15, windows 1.4. Renovation of retained elements: walls 0.30 (0.55 cavity fill), floor 0.25, roof 0.16.
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: 0.26 W/m²K
- Roof (all types, pitched or flat): 0.16 W/m²K
- Ground floor: 0.18 W/m²K
- Windows, curtain walling (new build limiting): 1.6 W/m²K (1.2 notional)
- Roof windows (new build limiting): 1.6 W/m²K (they fall under the Window row, Table 4.1 note 5); rooflights assessed horizontally: 2.2
- External doors (new build limiting): 1.6 W/m²K
- Party walls: 0.20 W/m²K (between dwellings)
Extension and Renovation U-Values
Extensions are NOT the same as new build: new elements in an extension must meet Table 4.2, which is stricter for fabric. Walls 0.18 W/m²K, floors 0.18, roofs 0.15 (one value for all roof types), windows 1.4 (or WER Band B), doors 1.4, rooflights 2.2. For renovation of existing thermal elements (Table 4.3), the “threshold” trigger values and “improved” target values are:
- Wall: Threshold 0.70, Target 0.30 W/m²K by internal or external insulation (0.55 by cavity fill)
- Roof (all types): Threshold 0.35, Target 0.16 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. The 2026 edition of Approved Document L (published March 2026, applying from 24 March 2027) requires low-carbon heating and, in most cases, solar PV for new homes; extension U-values are unchanged.