When it comes to heating, it really depends on what you need and certainly also on how it is sized. If you take a simple gas boiler, it doesn’t cost much. But our installers charge €39/hour and usually come in pairs. We have paid about €9,000-11,000 for the water and heating stuff in the house so far. And that’s without the heating itself, and the kids’ rooms kept the old radiators. As I said, friends of ours paid €24K for the solar thermal system on the roof, without pipes and radiators. They compared prices and get some money back through the KFW, but some things simply cost money. It’s not always just the materials that cost, but the working hours too. Here you really can’t compare and save much. Usually, you have to be glad if you can get craftsmen! And especially good ones, those are even harder to find.
And that we don’t know here whether and what needs to be done, I think everyone has already written about that.
So please report back when you know more and above all, I would also be interested in what the expert said about the little house! We also had a civil engineer with us who roughly estimated what it would cost and that was surprisingly accurate.