Cavity walls, generous lofts and standard plumbing make interwar semis the most forgiving heat-pump retrofit Britain has, which is why this pattern is now installed in the tens of thousands each year.
Before any kit conversation, the building tells you what is possible. A interwar semi (1920s–1930s) typically presents early cavity walls (often un-filled). Most also carry pitched tile roof. These facts shape every later decision about comfort.
The most common issue this property surfaces is cold rooms, followed by high bills. Any sensible plan addresses those first.
The constraint here is rarely the house. It is finding an installer with the design capacity to do the job at low flow temperatures rather than copy-paste the previous week's quote. A poorly sized system in a forgiving house is still a poorly sized system, and an interwar semi will hide that fact for a year before the bills tell the truth.
Insist on a room-by-room heat-loss calculation, not a rule-of-thumb sizing based on floor area. The cost of the survey is trivial compared with the cost of a 9 kW unit installed when a 6 kW would have been right.
Fill the cavity walls if the survey confirms they are empty and dry, top the loft to 300 mm, and the heat pump can run all winter at flow temperatures in the mid-forties. That is the regime where the running costs match or beat a gas combi on the same tariff.
The published cost range for this work lives in the cost registry, not on this page. The scope it covers is air-source heat pump, 7 kw, fully installed in a typical uk property.
An interwar semi with insulated cavities is the closest the UK comes to a textbook heat-pump house. The honest variation between quotes is rarely about the building. It is about the installer's design discipline, which is why the homeowners who shop around on competence rather than price tend to come away with the quietest system on the street.
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