Part L Compliance Calculator — U-Values & Thermal Performance

For new dwellings the notional targets (AD L 2021 Table 1.1) are walls 0.18, floor 0.13, roof 0.11, windows 1.2 W/m²K (limiting backstops in Table 4.1: wall 0.26, roof 0.16, floor 0.18, windows 1.6). For extensions and new elements in existing dwellings (Table 4.2): walls 0.18, floor 0.18, roof 0.15, windows and doors 1.4. For renovating retained existing elements (Table 4.3): cavity-filled wall 0.55, internally or externally insulated wall 0.30, floor 0.25, roof 0.16; replacement windows fall under Table 4.2 at 1.4. These are maximum values, lower (better) U-values are always acceptable.

SAP (Standard Assessment Procedure) is the government methodology for assessing energy performance of dwellings. A full SAP calculation is required for all new dwellings, major extensions and conversions. It considers U-values, air permeability, thermal bridging, heating systems, renewable energy and solar gains to calculate CO2 emissions and energy use.

Yes. Part L allows trade-offs through a SAP calculation. If one element slightly exceeds the target U-value, you can compensate by improving another element beyond its target. For example, an extension wall at 0.20 instead of 0.18 could be offset by a roof at 0.12 instead of 0.15. The overall building must perform no worse than one built to all target values.

Thermal bridges are points where insulation is interrupted (e.g. at junctions between walls and floors, around windows). They increase heat loss beyond what U-values alone predict. Part L accounts for thermal bridging through psi-values at junctions. Good junction details can reduce thermal bridging by 50% or more. Default values are very conservative — calculated values from accredited details are preferred.

Part L 2021 does not set a minimum air permeability for extensions, but the notional building for new dwellings assumes 5 m³/hr/m² at 50Pa. Better airtightness improves SAP performance and reduces heating demand. Below 3 m³/hr/m², mechanical ventilation (MVHR) is recommended to ensure adequate air quality.