Floor plan single-family house with separate apartment as a 3-unit house

Me again, I looked at some things again and read them carefully. What I noticed is:

How can the entrance area be optimized regarding daylight?

The upstairs staircase could benefit from a window in the east. I had thought of roof windows for the lower hallway, but that's where the upstairs already starts.

Apartment for tenants:

[*] Size: approx. 50 sqm; orientation was our old 2-room apartment here

It makes a difference whether two people in love/young couple share an apartment or a senior couple. The reasons are obvious: tolerance and ailments, hearing impairment, or described it very well recently :)

The parents (both in their early 60s) currently still live in their own house. Since the children moved out, much is empty

But they could, if they wanted, change rooms, avoid each other.

"Age-appropriate" fits better. And it does for this purpose, we think. Don’t you?

One has to say that my father is a self-employed farmer himself

For a former farmer probably less... Man, he may do the garden, but he's rather locked up there, isn’t he?!

Of course, further feedback on the rest of the plan, not concerning the tenant apartment, is also welcome.

We just wanted to have certain eventualities considered. This includes, for example, positioning the stairs so that we could divide the house into two residential units with ground floor and upper floor if we wanted to.

Maybe we're getting carried away with the thought of planning for every eventuality?

Yes, I think so. It is a stylish house, no question. But inside, the upstairs is really left aside. You have to go through a "hallway" to get to the children's room. I think, as a small child, I would be afraid to have to open the "apartment door" and then go through the place where the garages also are to get to the sleeping floor. And what if I sleep restlessly as a child? Then you have a very long and far way to check if everything is alright. I don't find that family-friendly. Small children do have to be checked fairly often. Passing through the chill lounge through a bottleneck at the kitchen is also not everyone’s thing. Basically, the house doesn’t really meet a need. It’s not a family house, for a yuppie house the kitchen is not well placed, the apartment might be suitable for an au pair, but it’s not senior-friendly for today's times.

I do not want to go into the draft in detail - not because it is bad, but because you took a wrong turn in the planning basics and therefore I recommend "starting from scratch." Assumptions / self-statements were taken at face value here, which I doubt. Especially on the point of how little one would get by "in old age," much self-deception with wishful thinking is involved. Before the joint architect meeting with the parents, first have joint sessions with a psychologist with the parents. Work out the reasons for reluctance why the parents do not want to move in immediately. Especially with an already existing grandchild and more planned, hesitation is nevertheless worth exploring. I expect valuable insights for the planning from this, which go far beyond structural details. As a management consultant – as someone to whom a vacancy with "usage" as an oversize office would already go against the grain financially, not only because of the special repayment contributions from a rental – I can assure you that even decent people see every superfluous square meter of a commercial property as a reliable target for clutter.

If more details are desired here, the bedroom on the ground floor is too narrow, the bathroom on the upper floor exactly above the living room?

I planned and then built a multi-generational house like this 20 years ago and am still very satisfied with it today.

Everything went as planned. However, we granted the granny flat 80 m2 and the best side of the house. The 3rd residential unit was even planned, but will surely never be realized. The kids are grown up and don't want to move out yet. They love living under one roof with grandma and grandpa.

I am in favor of multi-generational houses.

I am also generally pro multigenerational house. I am also building one myself (or converting one). But I think the principle should be to plan the apartments in such a way that you would want to move in there yourself at any time. And not think "at that age I don't have any more demands" etc. If you were two, without children, would you want to move into the granny flat now? In old age, the (necessary) requirements for comfort tend to increase rather than decrease.

In my opinion, planning a third residential unit in advance is never sensible. What is the probability that the (partly not even existing yet) children (at the OP) would later want to live in the same city or even in the parental home? If that should ever be the case, a new solution can be found in two decades.

Work out the reasons for the hindrance why the parents do not want to move in immediately. Especially with an already existing grandchild and others planned, hesitation is nevertheless worth exploring. I expect valuable clues for planning that go far beyond structural details.

In my opinion, a very important thought where all sides should be honest. Strictly following the motto: Say the bad things right at the beginning! I also find it just as important that the parents clearly take a position, because in turn they cannot necessarily expect that the young man will do it whenever it occurs to me someday.....
I hope you don’t take this the wrong way, but I mean it seriously FROM MY OWN EXPERIENCE, because such a long coexistence is nice but can also go wrong if it hasn’t been clearly clarified from the start. Often unspoken expectations or wishes simmer beneath the surface.....
Therefore, your parents should decide clearly, because you notice the twists you are currently turning in your planning, as it is supposed to cover all wishes and eventualities.
So this also applies to the parents: put your cards on the table and get rid of the subjunctive!

Work out the reasons for reluctance why the parents do not want to move in immediately. Especially with an existing grandchild and more planned, such hesitation is worth exploring. I expect valuable insights for the planning that go far beyond structural details.

Well, honestly: Besides the argument that the father is still an active farmer on his farm, no one wants to simply submit to and subordinate themselves to their children right after retirement—as we have all already said: it is no reason to go to a psychologist if one wants to feel comfortable in their old home for another 15 to 20 years without having to say "we are moving in with the children." Even if a grandchild plays a role, or especially then, one is glad to still live one's own life and not according to the pace of the young family.