Floor plan and extension of a house - Year built 1966

Wooden beam ceilings require generous support beams with appropriate bearings for such spans. Or some columns. Such large, open spaces are better achieved with concrete ceilings.

I agree. As I said, even with our concrete ceiling, we had to pay extra to get rid of a support post and achieve such a large open space. Honestly, I can't imagine that this can really be done with a wooden ceiling.

Sure, but then you might have a > 30 cm high KVH beam as a support beam or its counterpart in steel. Both then require corresponding bearings. And I suspect that more than one support beam is needed so that the ceiling does not swing too much. But that is just my layman's opinion. For such an ambitious renovation, I would consult an architect very early on before letting all the dreams grow too much only to bury them later.

Asked from a different perspective: What do you see as worth preserving about the house and why?
Put differently: Is it just a passion project or can it simply be done away with?

Asked from a different perspective: What do you see in the house as worth preserving and why?
Put differently: Is it just a matter of sentimentality or could it simply be done away with?

The floor plan fits our workflows very well. My wife is involved :). We save part of the demolition. There is more KFW funding available for renovation than for new construction. Even as a new build, I can imagine implementing this floor plan as it is. Can this be built as a new build at a comparable price?

Then a concrete ceiling might be more sensible. Probably not a big cost difference.

I honestly don't believe that you can renovate it for 400k, nor that you can build it new for that amount. I assumed the 400k in the original post were purely the renovation costs and that the purchase costs for the property are already excluded, right?

I am known for speaking uncomfortable truths: I consider the renovation for 400k a pipe dream. You are practically building new. You are only using the old (presumably uninsulated ground slab) and the old (technically catastrophic in terms of insulation) exterior walls. Everything future has to conform to the walls. You are not saving anything there.

Upstairs you are adding another floor, a new roof, outside you are putting every conceivable expensive climate gadget on the house that you once saw on Instagram.

I see this as neither a new build nor a "renovation" for 400k.