Failures
Wall U-value 0.18 W/m²K exceeds the maximum backstop of 0.18 W/m²K (AD L Vol 1, Table 4.1) — marginal non-compliance; no allowance for thermal bridging at junctions.
Warnings
No Part O overheating assessment provided. AD L 2021 cross-references AD O — required for new dwellings in England (§4.1).
Gas boiler heating system specified. While permissible under AD L 2021, gas boilers are incompatible with the Future Homes Standard transition pathway.
Passed Checks
Roof U-value 0.11 W/m²K ≤ maximum 0.11 W/m²K ✓
Floor U-value 0.13 W/m²K ≤ maximum 0.13 W/m²K ✓
Window U-value 1.2 W/m²K ≤ maximum 1.2 W/m²K ✓
Glazing ratio within 25% limit (estimated) ✓
AI Assessment
The design is marginally non-compliant with AD L 2021 due to the wall U-value sitting exactly at — but not within — the permitted maximum when accounting for thermal bridging at wall/floor junctions. The fabric performance is otherwise reasonable for a domestic extension of this size. The primary remediation action is to improve wall insulation to achieve a U-value of 0.17 W/m²K or better, and to provide an overheating risk assessment under AD O. The specified gas boiler is compliant with AD L 2021 but should be noted as a future liability if the client intends to sell or refinance after the Future Homes Standard comes fully into force.
Sample result · Oak Tree House, Birmingham · Part L 2021
Failures
Critical: Wall U-value 0.18 W/m²K exceeds FHS maximum of 0.15 W/m²K (FHS (ADL1 2026) §1.2a). Improvement required: reduce to ≤0.15 W/m²K.
Major: No heat pump readiness provision. FHS (ADL1 2026) §2.3 requires all new dwellings to demonstrate heat pump readiness — either via installed ASHP/GSHP or confirmed low-temperature heating system with space allocation for future heat pump installation.
Warnings
Minor: No Part O overheating assessment provided (FHS (ADL1 2026) §4.1). Required for England; assessor must confirm Criterion 1 (dynamic thermal analysis) is met.
Gas boiler specified: incompatible with FHS (ADL1 2026) primary heating requirements. Replace with ASHP or GSHP to achieve compliance.
Passed Checks
Roof U-value 0.11 W/m²K ≤ FHS maximum 0.11 W/m²K ✓
Floor U-value 0.13 W/m²K ≤ FHS maximum 0.13 W/m²K ✓
Window U-value 1.2 W/m²K ≤ FHS maximum 1.4 W/m²K ✓
Recommendations
Upgrade wall insulation to 150mm mineral wool (or equivalent) to achieve U-value ≤0.14 W/m²K — provides margin above the 0.15 threshold.
Replace gas boiler with air source heat pump. At 145 m² floor area, an 8–10 kW ASHP is appropriate. Confirm low-temperature emitter sizing (ΔT35) with mechanical engineer.
Commission dynamic overheating analysis (CIBSE TM59) and include in design pack submission.
AI Assessment
This design does not meet the Future Homes Standard (ADL1 2026) on two material grounds: wall fabric performance and heating system specification. The FHS marks a step-change from AD L 2021 — the wall U-value limit tightens from 0.18 to 0.15 W/m²K, and gas boilers are incompatible with the low-carbon heating requirement. Both issues are remediable at design stage without significant structural change. The recommended path is to increase wall insulation thickness by approximately 30–40mm and substitute the gas boiler for an air source heat pump; these changes will bring the design to full FHS compliance and should not materially affect programme or build cost at this stage.
Sample result · Oak Tree House, Birmingham · FHS (ADL1 2026)