Technically, you could use thermal solar panels & a very large thermal store under a house. Some of the houses in my area already heat their indoor swimming pools like this all summer & they then benefit from having a large thermal mass to keep their house warm in the autumn & winter. Think of it being like a storage heater.
The point here is that thermal solar panels (the ones that heat water) are something like 90% efficient so are by far the cheapest way of heating water, and a large insulated tank or two full of hot water doesn’t cost much to add to new builds.
However, as usual the most simple & obvious solutions are overlooked.