Heat pumps in London extensions: when they make sense and when they don't

From April 2025, new fossil-fuel heating systems in new builds are effectively banned under Part L of the Building Regulations. Existing dwellings retain the option to repair or replace like-for-like (gas boiler for gas boiler) but adding a new heating system in an extension increasingly defaults to heat pump.

Air source heat pumps (ASHPs) deliver heat at a Coefficient of Performance (CoP) of 3.0–3.8 over a London winter — meaning 1kWh of electricity produces 3.0–3.8kWh of heat. At current electricity unit rates around £0.28/kWh, this delivers effective heat at £0.075–0.094/kWh. Gas at £0.06/kWh and 90% boiler efficiency delivers effective heat at £0.067/kWh. Heat pumps remain marginally more expensive to run on grid-spot rates, but with PV solar, time-of-use tariffs or the Boiler Upgrade Scheme grant of £7,500 the economics tilt decisively toward heat pumps.

Building fabric matters more than the heat pump spec. A heat pump delivering 35°C flow temperature into oversized radiators or underfloor heating in a well-insulated home is efficient and comfortable. The same heat pump delivering 55°C flow into undersized radiators in a leaky Victorian terrace is inefficient (CoP drops to 2.0–2.5) and may not actually warm the home on cold days.

Best candidates for heat pumps: new builds (excellent fabric, designed-in low-temperature heating); extensions with underfloor heating (low flow temperature suits heat pump); full renovations with insulation retrofit (transformed fabric); modern flats with high-efficiency baseline.

Marginal candidates: older properties without insulation upgrade; properties heavily reliant on hot water (heat pumps deliver lower hot water flow than combi boilers, and cylinder storage adds complexity); properties with poor outdoor space for the external unit.

Outdoor unit placement is the single biggest issue in London. The unit is typically 1.0×0.8×0.3m, weighs 60–90kg, runs at 42–55 dB(A) at 1m. Position requirements: 200mm clearance behind, 600mm clearance to the front, mounted on a wall bracket or ground frame. Noise from neighbouring properties' boundaries: the planning permitted development rights require <42 dB at the nearest habitable window of the neighbouring property. For most London terraced sites, this means the unit cannot be on the rear elevation facing the neighbour's lounge window — sometimes requiring re-routing to the side return.

Permitted development for heat pumps was expanded in 2024 but with stricter location and noise requirements. Most installs proceed under PD; some sites need full planning. We assess at quote stage.

On larger London homes (200m²+) with high hot water demand or partial fabric upgrade, a hybrid system combining heat pump for primary heating with a smaller gas boiler for peak demand and hot water can deliver lower running costs than either system alone. The heat pump handles 70–85% of annual heating load efficiently; the boiler handles cold-snap peaks and hot water peaks where the heat pump would struggle.

Hybrid systems remain compliant under current building regulations for existing dwelling extensions. They are not permitted for new builds. We've installed 12 hybrid systems in the past two years, all with significant cost savings versus gas-only and significant comfort improvement versus heat-pump-only in older homes.