What This Requirement Covers
Building extensions must comply with Approved Document L to ensure they do not significantly increase the energy consumption of the overall building. The requirements apply to all extensions, from small single-storey rear extensions to large multi-storey additions.
Key Requirements
New Elements in Extensions
All new thermal elements in the extension must meet minimum U-values
- Walls: 0.28 W/m²K
- Floor: 0.22 W/m²K
- Roof (pitched, insulated at ceiling): 0.16 W/m²K
- Roof (flat): 0.18 W/m²K
- Windows and roof windows: 1.4 W/m²K
- Doors (glazed >50%): 1.4 W/m²K
- Doors (solid or <50% glazed): 1.0 W/m²K
Fixed Building Services
- Heating in the extension should be from the existing system (extended) or a new system meeting Part L efficiency standards
- New lighting must be energy efficient (LED or equivalent)
- Ventilation must comply with Part F
Consequential Improvements
For larger extensions (over 100 m²), the existing building may need to be upgraded
- Upgrade existing boiler if non-condensing
- Top up loft insulation to current standards
- Fill existing unfilled cavity walls
- Upgrade hot water cylinder insulation
Area Threshold
- If the total area of windows, doors, and roof windows in the extension exceeds 25% of the extension floor area, the extension must be assessed using a SAP calculation or the compensatory approach (trading off better fabric in one area against more glazing)
Practical Compliance Tips
- Use the Part L Volume 2 tables to check all new elements meet the minimum U-values
- If the extension has a large glazed area (bifold doors, roof lanterns), use the compensatory approach to demonstrate compliance
- Extend the existing heating system where possible; a new heating system must meet current efficiency standards
- Ensure insulation is continuous between the extension and the existing building; gaps at the junction are a common failure
- Air tightness of the extension affects the overall building performance; seal the extension carefully
- Submit the Part L compliance evidence to Building Control with the full plans application
- For larger extensions, budget for consequential improvement works on the existing building