Heat pumps are not strictly mandated under Part L 2026, but they are the only realistic compliance route given the carbon targets. From 24 March 2027 the great majority of new dwellings in England will use air source heat pumps.
Why ASHPs dominate FHS-compliant heating
Air source heat pumps deliver ~3 kWh of heat per kWh of electricity (Seasonal Coefficient of Performance of 3.0 typical). Combined with the low-carbon grid this delivers ~0.03 kgCO₂/kWh of heat — versus 0.21 kgCO₂/kWh for natural gas. ASHPs are the only widely-deployable technology that hits the FHS targets.
Sizing rules
Heat pump output is sized to peak heat loss at the -2°C (or local design temperature) external condition. Typical sizing for an FHS-compliant 3-bed semi: 5–6 kW ASHP. 4-bed detached: 6–8 kW. The hot water cylinder needs to be sized to ~180–250L depending on bedroom count.
Integration with solar PV
Solar-paired ASHPs run efficiently when the sun shines — and the compliance calculation credits the PV output. A 5 kWp solar array paired with a 6 kW ASHP and a 5 kWh battery is the FHS notional building for a typical 3-bed semi.
Cylinder requirements
Hot water cylinders for ASHP must be larger and lower-pressure than gas-fired equivalents. Typical specification: 180L unvented cylinder for a 3-bed, 200L for a 4-bed. Pre-plumb for solar thermal is not required (PV diverter to immersion is more cost-effective).
Cost and supplier ecosystem
ASHP units from Daikin, Mitsubishi Ecodan, Vaillant aroTHERM, Samsung EHS and Grant Aerona are widely available at volume. Bulk procurement pricing for developers runs £2,200–£2,800 per system installed.