Cultivation Planning / Change

On the other hand, bedroom, children's room and bathroom in the basement, and the rest of the apartment on the upper floor - doesn't sound very appealing either. And it will cost a fortune...

I ask you, this is really crap.

Plain speaking: You twist and turn like an eel because you don’t want to die a horrible death, yet you still have to die. There are reasons for and against everything, all supposedly compelling.

If you really see it that way, you won’t get anywhere. Then look for a suitable existing property. I consider new construction financial harakiri. But the dithering has now taken absurd forms, without meaning to offend you.
That can’t be good for family peace if there’s a compromise that includes something for everyone but makes no one truly happy. Or where one can keep everything the same and another bends until he almost breaks.

One has to say at this point: the OP is obviously under enormous emotional pressure. On one hand from his own family, who are standing on each other's feet and want more space. On the other hand, he feels emotionally so obligated to the parents-in-law that he does not want to impose even the smallest inconvenience on them. He tries to please everyone somehow but forgets himself and every rational logic in the process. I'll throw Plan D into the room: You buy the parents-in-law a small condominium and use the huge house for yourselves. You need to emotionally detach yourself a bit; in the long run, this causes stomach ulcers.

One has to say at this point: the OP is obviously under enormous emotional pressure. On the one hand from his own family, who are practically on top of each other and want more space. On the other hand, he feels at least emotionally so obligated to the in-laws that he doesn’t want to impose even the smallest inconvenience on them. He tries to please everyone somehow, but forgets himself and any rational logic in the process. I’ll throw Plan D into the ring: You buy the in-laws a small condominium and use the huge house for yourselves. You need to emotionally detach a bit; in the long run, this will cause stomach ulcers.

I see Plan D fading away.

The grandparents serve as daycare replacement, and the wife probably doesn’t want the parents to live very far away.

I don’t want to harp on your wife and your in-laws... but you should get all parties together and make it really clear what it means for you to have to spend another 250k€ for a "compromise solution" only. Because it will weigh on you continuously and not on your wife or your in-laws. Because only then will everyone start screaming if you were to lose the house due to some bad coincidence.

I really think it's a bad habit how the parents are shuffled back and forth here like cattle. Can you just leave them alone in peace?

I'd rather throw another plan into the room. It will never be nice, and you can't avoid the long corridors because of the stairs issue even with the architect's design. That's probably a tough pill to swallow, for which I can only say enjoy your meal.

Water disposal in the parents' bathroom could, however, become interesting.

I really find it rude how the parents are being pushed around here like cattle. Can you leave them alone?

It's clear, parents are saints, children on the other hand are absolutely negligible.
In your little world that's how it is.
But that's not the topic here, the OP needs a solution and not someone who gets him more of his drug.
What does the OP do with a 300m² house when the children have moved out and the parents are under the ground?

I really find it rude how the parents are being pushed around here like cattle. Can you leave them alone in peace? QUOTE]

What is wrong with a stairlift and a balcony on the upper floor? They are not demented or in need of care and can certainly decide for themselves if it would be okay.

By the way, a friend had the same solution.
- He inherited the house from his parents.
- Added a living room downstairs and converted the ground floor for himself, as well as a terrace upstairs on the upper floor for dad and mom. Guess what, everyone is happy (without a stairlift).

So no pushing around.

You just have to free yourself from mental constraints.