Understanding Building Regulations Part L: Energy Efficiency
Part L of the Building Regulations sets out the requirements for energy efficiency in new and altered buildings in England. It's relevant to any extension, loft conversion, replacement window installation, or significant renovation project — because improvements to the energy performance of the building fabric are a legal requirement, not an optional extra, whenever notifiable works are carried out.
What Part L Requires
Part L is divided into two volumes: Part L1A (new dwellings) and Part L1B (existing dwellings). For most domestic renovation and extension projects, Part L1B applies.
Part L1B requires that whenever notifiable works are carried out on an existing dwelling, the opportunity must be taken to upgrade the energy performance of the building fabric where it is reasonable to do so. The specific requirements include minimum U-values (thermal transmittance values) for new or replacement building fabric elements — walls, roofs, floors, windows, and doors — and requirements for the efficiency of heating system upgrades.
Minimum U-Values Under Current Part L1B
Current minimum U-values for new fabric elements in existing dwellings (as set out in the 2021 Part L1B Approved Document) include: walls: 0.18 W/m²K; roofs: 0.15 W/m²K; floors: 0.18 W/m²K; windows: 1.6 W/m²K; doors: 1.8 W/m²K. Replacement windows and doors installed on their own (not as part of a wider extension project) must also comply with these values.
The 25% Rule and Consequential Improvements
Where a domestic extension exceeds a certain size threshold (defined as a floor area greater than 25% of the existing floor area), the regulations require not only that the extension meets the relevant U-value standards, but also that consequential improvements are made to the existing building's energy efficiency — for example, improving insulation in the existing roof where this is technically feasible and cost-effective.
This provision is sometimes called the "25% rule" and is often misunderstood. Your contractor and Building Control officer can advise on whether it applies to your specific project.
EPC and Energy Performance
An Energy Performance Certificate (EPC) is required when a property is sold or let. It rates the property's energy efficiency from A (most efficient) to G (least efficient). A significant refurbishment or extension that improves insulation, upgrades windows, and installs a more efficient heating system typically improves the EPC rating — which has commercial value for both sale and rental properties.
If you're planning a major renovation, it's worth getting an EPC assessment before and after the project to quantify the improvement and to identify any quick wins that could be incorporated into the project at low marginal cost.
07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk