A passivhaus uses 75% less energy for space heating than UK new build requirements, because the houses are so well insulated and can make the most of the passive solar gain (technical term for sun coming in through windows).
We now know from the results of our technical feasibility study that ground source heating would not be cost effective here, because of the high installation cost of the infrastructure. So we are keeping it simple, which also means less embodied carbon in the construction!
It looks likely that we could use a thermostatically controlled simple electric room heater, although there is still potential for air source heat pumps for water heating.
But of course if the tenants don't want to switch on their heaters, they could invite the neighbours round for some dancing which would also warm up the house for free!