A recent factfilled post on facebook in regard to Air Source Heat Pump gave a price of between £9000 to £15000 per household to upgrade ( so virtually as new installation), based on trials already completed on various sized homes. Yes a large and remote from neibours, space for the unit is required for the heat exchanger which is basically a large radiator and even larger fan to drive the outside air through it.So yes very limited use on many new homes that are very close together.