Approved Document L (Energy Efficiency): What Homeowners Need to Know

Part L is split into two volumes:

• Volume 1 (L1A/L1B): dwellings — covers new dwellings (L1A) and extensions/alterations to existing dwellings (L1B).
• Volume 2 (L2A/L2B): buildings other than dwellings.

For homeowners, L1B is the most relevant document. It sets minimum fabric standards for extensions and requires that any new or replacement building elements (walls, roofs, floors, windows) achieve at least the specified U-value.

A U-value measures how much heat passes through a building element (wall, roof, floor or window) per square metre per degree of temperature difference. Lower is better — a lower U-value means better insulation.

Current minimum U-values for domestic extensions (Approved Document L1B, 2022):

• External walls: 0.18 W/m²K
• Party walls: 0.20 W/m²K
• Floors: 0.18 W/m²K
• Roofs: 0.15 W/m²K
• Windows, doors and rooflights: 1.4 W/m²K for windows; 1.4 W/m²K for doors (with more than 60% glazing); 2.2 W/m²K for rooflights

These are minimum standards — a good designer will often specify better performance to reduce running costs and carbon emissions.

The Future Homes Standard will require new homes in England to produce significantly fewer carbon emissions than under the 2013 Part L standard — a reduction of approximately 75–80%. The key changes for homeowners and developers include:

• Tighter fabric U-values (walls approaching 0.15 W/m²K, roofs 0.11 W/m²K)
• Very low air permeability targets (requiring MVHR ventilation — see Part F)
• No new gas boilers in new-build homes; low-carbon heating (typically heat pumps) required
• Photovoltaic (solar) panels likely to be required on most new dwellings

The FHS applies to new dwellings. Extensions to existing homes will continue to use Part L1B, though the standards are expected to tighten incrementally.

For new dwellings, a SAP (Standard Assessment Procedure) energy assessment is required to demonstrate compliance. A SAP assessor models the energy performance of the dwelling and produces an Energy Performance Certificate (EPC). The SAP calculation must be submitted to building control.

For extensions to existing homes, a simplified approach is usually acceptable: demonstrating that each building element meets the minimum U-value standard. However, for large extensions or where the extension is significantly larger than the existing dwelling, a more detailed assessment may be needed.

The most cost-effective ways to meet Part L in a typical home extension:

• Walls: 100 mm cavity with full-fill mineral wool (or 75 mm PIR foam board) achieves 0.18 W/m²K.
• Roofs: 100 mm PIR board between joists + 50 mm PIR below achieves 0.15 W/m²K.
• Floors: 100 mm PIR below the floor slab achieves 0.18 W/m²K.
• Windows: standard argon-filled double glazing with a low-e coating achieves around 1.2–1.6 W/m²K. Triple glazing (0.8 W/m²K) significantly exceeds the minimum.
• Thermal bridges: junctions between elements (wall/floor, wall/roof) must be designed to minimise heat loss through the structure. Approved Document L references Accredited Construction Details (ACDs) for standard junction solutions.