We are only a two-generation house here, but the elderly live downstairs and the young live upstairs ;-) We are still planning a staircase to the garden for direct access and my husband is planning a pulley system for when he's feeling lazy and the kids will have fun with it too (at least I always had fun with it as a child at my friend's house where the family lived on the 3rd floor :-D)
I would also make 2 smaller apartments downstairs and 1 large one upstairs.
We had a lot of time to think about this and after 5 years we said that this feels very comfortable for us and the planning for renovation and reconstruction also makes it easier.
Hmm, she’s probably just thinking a bit more. It’s only Wednesday today after all. You can take 3 days for an answer, especially when it’s such an important decision. She also has to digest the input first. I can understand that. And maybe she also has other things to do on the side.
Out of gut feeling, I would do the following: have the existing house inspected, renovate if necessary, and place a small modular house for the "young family" in the garden. If possible.