The ground floor and upper floor should therefore be decoupled in terms of noise
That sounds nice with the "decoupling," but depending on the room layout and also with a concrete ceiling in between, you will hear the roommates and especially even better if the relationship there is not good. You don't have a really large plot of land, and now several parties are also supposed to benefit from it in some way? So the jack-of-all-trades also continues to live outside?
These thoughts are basically not foreign to me, but back then my parents were there, who lived in the apartment for a long time. I am building a house for myself and for now in the version "nice and maximally comfortable" (really individual comfort).
Planning the future in this way seems less sensible to me and indeed more expensive than you might calculate here.
It is already showing that for cost reasons you will have to switch to a "simpler" version when choosing the controlled residential ventilation, and the same will happen to you at other equally important points, that the budget pinches. Why build for a naturally completely uncertain future while paying less attention to current needs?
Ideally, of course, the kitchen could be separated with a sliding door if needed,
Why, what situation do you want to cushion with that? I had it exactly like that at the time. The door(s) were expensive, never used, this was planned more in times of missing or weak extractor hoods or when you didn't want to see the working woman. You value guests and spending time with them, and then something like that?
I would really like the seat window.
Do you really read those books there and, very importantly, have you ever sat in such a place for a longer time? I would examine that very carefully, but I can understand the spontaneous wish alone. I just always find it a pity when money is then missing elsewhere and for truly comfort-giving things.
because I have the feeling that I am "surrounded" by neighbors to the south and west
And then you voluntarily want to bring someone into the house?
The house should fit as many changed life situations as possible
The much-discussed, jack-of-all-trades here
2 children and also want to live in the house in old age and then rent out the upper floor or the upper floor can be used for a caregiver)
Actually, I hear such plans again and again but despite somewhat older age I know no one who has done it that way.
Daily routine: wife comes home around 2:00 pm and I around 6:00 pm on weekdays. During the week I do a lot of home office. On weekends we like to have guests and there should also be the option that guests can stay overnight with us, because relatives sometimes come from other federal states.
Yes, that exactly is individual need in today's life and I would enable that as well. But why then separate the kitchen, especially then you want to be together and maybe even cook and chat.
Also the question whether I have to keep a separate apartment for that, especially if I have a manageable budget and limit myself otherwise. We have, for example, a multifunction room (not a junk room) where various guests sleep and directly opposite a bathroom that they can also use alone, otherwise this belongs to our daily life.
I don’t imagine the separation of heating and electricity to be so complicated,
Such a separation not only causes considerable costs, but also somewhat limits free planning, which in turn negatively affects the present. For example, you outsource the staircase exactly for this reason, which otherwise would not be necessary and a nice staircase in the open plan area certainly doesn't have to be a disadvantage.
I only remember the comment from several relatives who say that the house has become too big for them without children and they have too much work, hence my idea. So I don’t find it that "nonsense."
But that is rather small talk that usually wouldn't withstand a serious and open discussion (see here). There are also people who never or only under pressure change something in life but also people who adjust things in life to their new situation and take matters into their own hands when needed or circumstances arise. Foreseeing all this requires several crystal balls. Here you meet other people and the answers are different, but here costs and necessity are also carefully considered and discussed openly and clearly.
You are allowed to build that and certainly find a company that implements it and maybe even people in your environment who find it good. Having such thoughts with the first house I also consider understandable at first and useful within brainstorming. But at some point, you should stop insisting on these positions even against the most factual advice, because it can become very expensive and lead to a poor result.
I recommend you build a single-family house with a nice floor plan for you/you now and with room for children/hobby or a multifunction room also for guests. Then nice rooms for shared time with exactly those guests and family you mentioned. Then also enough space in the garden with privacy, and if you really have the money for it, then treat yourself to the air conditioning, the fancy terrace roofing, the comfortable garden furniture and much more.
Ergo: back to square one. We have done/had to do that countless times, and so far the result suits us quite well today.