You wouldn't use a potable water tanked heater for underfloor heating anyway. You'd use a boiler. Or a completely separate rigged water heater. The water in a radiant heat system is a closed loop.

For the hot water needs of the dwelling, a tankless would be perfect if it's sized right. The incoming water temperature in winter tells you how much rise you need from a unit. And how many items that you want to operate at once will tell you how many GPM that you need. I've had a tankless for 18 years now, and we are a "serial use" household. No two showers at once. One after the other. Then we can do the laundry or wash the dishes after we get out of the shower. So, our need for high GPM just isn't there. We still need a lot of BTU's though to combat the 40 degree winter water.

You've got to do the math for the size you need, but your situation is the perfect one for a tankless.

A condensing high-efficient combi boiler would be in order here.

"A condensing high-efficient combi boiler would be in order here."

HI efficiency investment for occasional use?

"HI efficiency investment for occasional use?"

"become a permanent residence for aging parents. It will have a kitchen (sink & dishwasher), bathroom (sink & shower), plus a utility sink & accommodation for a small clothes washer in the garage portion."

I would say with the desire of infloor heat, the structure will be built in a freezing climate, it will have fixtures, washing machine, water lines, traps, all that can freeze. And someday could be used a a residence, I wouldn't call it occasional anymore, at least not the domestic water & heating system, it will be needed full time. You may use it occasionally to start, but the heating system can't operate occasionally.

So yes, A compact dual operation hi-efficiency appliance would make great sense.