Part L Building Regulations 2021
Part L of the Building Regulations sets standards for the conservation of fuel and power in buildings. The 2021 edition introduced significantly improved U-value requirements.
Note: These requirements came into force on 15 June 2022 for new dwellings and 15 June 2023 for existing dwellings.
U-Value Requirements Summary
New Dwellings (Part L1A)
| Element | Limiting U-value (W/m²K) | Notional U-value (W/m²K) |
|---|---|---|
| External Wall | 0.26 | 0.18 |
| Party Wall | 0.20 | 0.00 |
| Floor | 0.18 | 0.13 |
| Roof | 0.16 | 0.11 |
| Windows/Doors | 1.6 | 1.2 |
| Rooflights | 2.2 | 1.7 |
Existing Dwellings - Extensions (Part L1B)
| Element | U-value (W/m²K) |
|---|---|
| External Wall | 0.28 |
| Floor | 0.18 |
| Pitched Roof (insulation at ceiling) | 0.16 |
| Pitched Roof (insulation at rafter) | 0.18 |
| Flat Roof | 0.18 |
| Windows/Doors | 1.4 |
Key Changes from 2013
- 31% improvement in fabric energy efficiency for new homes
- 27% reduction in CO2 emissions target
- New requirements for overheating mitigation
- Improved airtightness standards
- Updated thermal bridging approach
Compliance Routes
1. Elemental Method
Each building element must meet the limiting U-values. This is the simplest approach but doesn't allow trade-offs between elements.
2. Target Fabric Energy Efficiency (TFEE)
The dwelling's total fabric energy efficiency must not exceed the target calculated using notional U-values. This allows better-performing elements to compensate for others.
3. Target CO2 Emission Rate (TER)
Calculated using SAP, the dwelling's emission rate must not exceed the target. This allows the most flexibility including services and renewable energy.
Common Compliance Issues
!Thermal bridging at junctions not properly detailed
!Party walls assumed to have zero heat loss incorrectly
!Floor U-values calculated without proper edge insulation
!Air permeability not meeting the 5 m³/h.m² target