... We decided that improving our building to full Passive House standards was just not doable, even if we were totally unrealistically "generous" financially, so we built a Passive House extension instead. That contains Kitchen-Diner, Snug and master & family bedrooms upstairs, and needs close to no heat input in Winter. In fact we run it at a toasty temperature, because we already had central heating [infrastructure] for the main part of the house and it only needs the smell of an oil rag ... but even though we have increased the foot-print of the building by 40%, and we heat the Passive House extension to shirt-sleeves temperature, our overall winter fuel bill has fallen by 50%. We run the main part of the house cooler, because we don't have to sit in it, except when entertaining (and I have always preferred to do that in the Summer when we can sit outside ... so that is "not much", and we have enough room in the kitchen for dinner guests).
Passive House brings lovely even temperature, no draughts - neither poor fitting windows / trickle vents, nor convection currents - no corners of rooms which get no air flow - and then grow mould / whatever - and no winter coughs & colds anymore, a benefit of Passive House even-temperature and mechanical-ventilation that I wasn't aware of when we started down that route.
If anyone is interested, and wants to come and have a look, you'd be very welcome. Just drop me a PM.