@christineburns @scotdowser My current dwelling here in the US has the outdoor units for each apartment perched on every available inch of roof space, although the heat here is very much still gas at the moment. (They installed the heat pumps without reversing valves for some strange reason, so they only work in "cooling" mode... Not that I really mind, given that my computers put out enough heat to keep the place "rather uncomfortably warm" without the thermostat's assistance, even in the dead of winter, when combined with my downstairs neighbors seemingly having the thermostat preferences of a lizard)

Though it is possible to have multi-head systems for larger flats or communal heating, so you could end up needing fewer outside units than you have apartments in the building.