Guide to Part L and Part O of the Building Regulations

The 2021 uplift to England's Building Regulations introduced significant changes to Part L (Conservation of Fuel and Power) and Part O (Overheating), which came into force in June 2022. If you are building an extension, a new dwelling, or carrying out work on heating systems or windows, understanding what these regulations now require is essential.

Part L of the Building Regulations governs the energy efficiency of both new and existing buildings. For new dwellings, the 2021 uplift increased the energy efficiency standard by approximately 31% compared to the previous Part L 2013. For extensions and material alterations to existing homes, Part L requires the work itself (and any consequential improvements) to meet minimum U-value standards for walls, floors, roofs, windows, and doors.

The 2021 Part L uplift is sometimes called the 'Future Homes Standard Interim' — it is a stepping stone to the full Future Homes Standard, which aims to make new homes zero-carbon-ready by 2025.

— UK Government, Department for Levelling Up, Housing and Communities

A U-value measures how quickly heat passes through a building element — the lower the number, the better the insulation. Under Part L 2021, new extension walls must achieve a U-value of no more than 0.18 W/m²K, roofs 0.15 W/m²K, and floors 0.18 W/m²K. New windows must achieve 1.2 W/m²K or better. Your builder and building control officer will check compliance with these standards.

Part O is new in the 2021 package — it addresses overheating risk in new dwellings, which has become a serious issue as UK summers become hotter. It requires new homes to be designed to limit solar gain (using shading, glass orientation, and materials) and to provide adequate ventilation for cooling. Part O applies to new dwellings; it does not currently apply to extensions, though good design should consider overheating regardless.

If you are building an extension, your builder must demonstrate Part L compliance — typically through a simple calculation submitted to building control. If you are replacing windows or doors in the existing property as part of the work, those must also meet current standards. Replacing a boiler now requires the new boiler to comply with the latest efficiency requirements, and new heating systems in new builds cannot be gas-only from 2025 under the Future Homes Standard.