What are the Part L 2021 U-value targets?

For new dwellings: walls 0.18, floor 0.13, roof 0.11, windows 1.2 W/m²K. For extensions: walls 0.26, floor 0.22, roof 0.15, windows 1.4. For conversions: walls 0.30, floor 0.25, roof 0.18, windows 1.6. These are maximum values — lower (better) U-values are always acceptable.

What is SAP and when is it needed?

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.

Can I compensate poor performance in one element with better performance elsewhere?

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, a wall at 0.28 instead of 0.26 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.

What is thermal bridging and how does it affect compliance?

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.

What air permeability is required?

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.

TradeCalculator

Part L Compliance Calculator — U-Values & Thermal Performance

Check building fabric U-values, air permeability and thermal bridging against Part L 2021 targets for new dwellings, extensions and conversions.

Building type

Wall U-value

Floor U-value

Roof U-value

Window U-value

Air permeability

How We Calculate This

This calculator compares your proposed U-values against Part L 2021 target values for the selected building type.

Element-by-element check

Each fabric element is checked against its target. Passing all targets indicates likely compliance, but a SAP calculation is needed for final confirmation.

Overall compliance

If any element fails, compensation through improved performance elsewhere may achieve overall compliance via SAP/SBEM calculation.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.