New construction project single-family house in the Bergisches Städtedreieck

Yes, that is also their wish.

That is nice. What you call it in the end doesn't matter. The fact remains that if the parents are to have their own little realm, then it must also be planned in such a way that one would want to live there. A bedroom where a wardrobe and a bed fit, a shower that works for seniors, and a living area that can be nicely zoned. The dimensions will probably be quite decisive here in the end, since the plot for 6 people is rather compact. To be precise, that is the first thing I would ask about, all of it.

then it must also be designed so that one wants to live there.

Yep.

A bedroom where a wardrobe and a bed fit, a shower that works for seniors, and a living area that can be nicely zoned.

However, this line reads more like a nursing home and not like age-appropriate living space in a single-family/two-family house.

Upstairs, behind the railing (approximately in the middle of the stairs), there is an entrance door to the storage room. The entire staircase does not need to remain free of obstructions. The staircase length was taken from the app and has a normal length (according to the measurements from the app: 3m long, 1m wide, 14 steps).

I am not a professional, so it should only be about ideas to have a rough idea of the direction. The absolute measurements do not play such a big role here.

However, the fundamental factual accuracy remains indispensable. Jumping over the railing in the middle of the stairs and branching off into the storage room is not a question of exact centimeters, but of essential feasibility. Nevertheless, the drawn staircase is considerably wider than 1 m.

If the planned floor area is too small, what would be the maximum possible dimensions of the house? Or 2.5 stories? Or a basement?

Thanks already to everyone for the valuable tips. I will discuss it with an architect or planner. The reason why the grandparents are roughly allocated these square meters is quite simple. They don't have more now either, and that's how we proceeded. The only thing that is bigger in their apartment is the hallway and a third room that was used so far to let the grandchildren sleep there when they were visiting. Since that will no longer be the case, only two rooms and a bathroom remain. And in the new design, there are even more square meters than they currently have available.

We grew up as four people in 42 sqm and did not complain. We are not in a "wishful thinking" mode here and now, but rather "this is how it is," and we try to make the best out of the situation including everyone involved. We cannot build the Denver Clan villa here, where there is space in abundance...

Even today, I see at relatives and friends who live with three generations under one roof how they live. The office in a new building is 4 sqm (!) or the third child has to live in the basement...
Seen all that personally. And since we were lucky enough to get this plot in the location we wished for, I am now trying to make the best out of it.
Therefore, thanks to everyone who is making constructive suggestions here, because we need them. If everything would work out according to a picture book, I wouldn’t have reached out to you all here in the forum.

Or 2.5 floors?

Yes, that would be possible. Basement, no. I find it too expensive in relation to what it offers. Especially since we have a high groundwater level here. It would have to be absolutely sealed.