Cost planning - core renovation of farmhouse / country house

I have learned, however, that in the best case you chisel out the timber framing and simply refill it with Ytong and plaster it. It actually went relatively quickly.

Ytong and timber framing do not go well together. This can lead to severe moisture damage, as has now been determined. It's not so simple. :)

The plot on which the mansard roof stands is so large (and mostly flat) that one could consider buying the thing, building a NEW house next to it and living in it, and then in a second step thinking about how to renovate the old building to make it rentable. :D With the renovation costs, that’s not as far-fetched as I initially thought.

I still have one question: If I buy a very old building with the intention to completely renovate it, meaning I want to redo everything anyway – what should I pay special attention to during the viewing?

I would pay very close attention to which compromises I could live with. Even with a full renovation, you can't simply redo everything. The load-bearing walls (or the statics in general) are the best example of this. If the structural engineer says that the upper floor cannot support a tub, then you have to be able to live with bathing on the ground floor. The room layout should ideally already fit as it is. Window positions and window sizes should be to your liking.

I would listen carefully to the staircase and the floors regarding how loud footsteps are and how loud a jumping child becomes. The basement would be important to me, whether it can be used for something or only for storing potatoes. Can the attic be converted and used, or does some wooden structure of the roof block all the space?

Otherwise, you can't see much if everything is going to be new anyway. Only an expert can help with that.

Oh yes: continuously calculate whether demolition and new construction might not be the cheaper option. You are not on site, so you have to have everything done. That costs money.