What FHS mandates
No fossil-fuel heating (so no gas boilers in new dwellings). Air-source or ground-source heat pump as the heating source. Fabric performance broadly aligned with notional dwelling U-values: walls 0.18, roof 0.13, floor 0.13, windows 1.2 W/m2K. MVHR or equivalent ventilation strategy. PV typically required to meet primary energy target[FHS].
Cost uplift over 2021 interim
Heat-pump system (ASHP, cylinder, controls, low-temp emitters): £8k-£14k uplift over a comparable gas-boiler system. Fabric upgrades (insulation thickness, airtightness detailing): £25-£60/sqm. MVHR system: £4k-£8k. PV array: £4k-£8k. Total uplift commonly £80-£180/sqm depending on starting baseline.
Transition for KDR projects
Applications submitted before the transition deadline can build to the 2021 interim Part L; applications after must meet FHS. The transition deadline depends on the final implementation date (originally 2025; revised guidance expected late 2026). Check the deadline in force at planning submission date.
Offsetting grants
Boiler Upgrade Scheme grant £7,500 toward an ASHP installation, available to owner-occupiers in England and Wales. Some Local Authorities run additional EWI grant schemes. Self-build CIL exemption is unaffected by FHS.