Only count the surfaces with cold on the other side. A mid terrace has half the exposed wall of a detached house the same size, and needs roughly a third less heat.
Heat flows down a temperature difference, and across a party wall between two heated homes there is barely any. The same goes for the ceiling under a heated flat and the floor above one. Sizing on floor area alone — or costing every house as though it stood alone — puts the same number on a detached house and a mid terrace, when the honest spread on one 110 m² house is about 3.6 to 6.2 kW. On a heat pump that is the difference between a 5 kW unit and a 7 kW one, and the oversized one does not merely cost more to buy: it cannot turn down far enough to match a mild day, so it short-cycles and runs at a worse efficiency than the one that fitted.
It assumes the neighbour heats their home. An empty flat next door, a holiday home, or an unheated stairwell puts a real temperature difference across that wall again — and an uninsulated party wall with a cold cavity can also lose heat upwards into the loft, which is the party-wall bypass. Treat it as nothing for sizing, not as a wall that can never matter.
Estimate with the rule, then check it against the calculator that models it properly.
Open Heat Loss & Heat Pump Sizing →