Part L U-Values at a Glance (England)
Part L U-Values at a Glance (England)
New Build (W/m²K)
| Walls | 0.26 |
| Floors | 0.18 |
| Roofs | 0.16 |
| Windows and doors | 1.6 |
Extensions (W/m²K)
| Walls | 0.18 |
| Floors | 0.18 |
| Roofs | 0.15 |
| Windows | 1.4 |
Replacement (W/m²K)
| Windows and doors | 1.4 |
| Fire doors | 1.8 |
| Walls | 0.30 |
| Roofs | 0.16 |
| Floors | 0.25 |
Key Points
- • These are limiting values (worst allowable) — the notional dwelling uses better values
- • England only — Wales, Scotland and Northern Ireland have different figures
- • U-values must be calculated, not guessed — use manufacturer data
- • Building control will check insulation installation, not just specification
- • Air tightness: max 8 m³/hr/m² at 50 Pa (new builds)
Common Traps
- • Extension walls (0.18) are tighter than new build (0.26) — this catches people out
- • Thermal bridging at junctions reduces real-world performance
- • Insulation compressed or gapped = actual U-value worse than calculated
- • Party walls between heated spaces still need edge insulation at ground floor
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