Floor plan of a two-family house (side by side) on a south slope

the terrace has slipped onto the wrong floor in the plan

Thank you very much! Now it makes sense!
First of all: I find the house more or less very nice, also very well thought out in terms of structure and room layout, etc.
You just have to live with the fact that the bedroom is close to the front door, and so on. Ultimately, it’s about the optimum, which includes compromises.
Parents-in-law:
I would forgo the pantry there. There is a large utility room anyway. The gained space could be added to the open living area or kitchen. In my opinion, the kitchen here is a bit cramped. I would also swap the living room and kitchen here, so that everything appears more spacious. Also, forgo a U-shaped kitchen.
If you place the entrance to the north, then you have more bathroom space and possibly also room for laundry.
Your apartment:
Clearly: plan a fixed staircase to the attic. Install a window in the attic in the gable.
Have the door to the utility room open outwards. We have exactly such a room, and the door there is annoying.
Since I’m a critic of those pantries accessible over two tall cabinets, where no one even drags in a crate of drinks, I say: plan the pantry next to the stairs!
Basically, I find the sides of the stairs too narrow for the TV corner, because it feels like it kicks you out of the sofa. I find long and narrow negative, instead short and wide better. In this respect, I would experiment a bit here to see if it works better/differently. A kitchen by the terrace would also be nice here.

First of all, I like the idea of togetherness, also because I personally have lived like that with my parents and would do so again and again. For us, it used to be one on top of the other, this here seems better. We never had any problems, on the contrary, but nevertheless or precisely because of that, I would also pay attention to maximum possible independence or separation in detail; equally for both sides. At some point, things can also change, and then a clear separation really pays off. Therefore, it should be clarified whether the shared heating room could not only be accessible from the outside. I recently had various problems there after the change, which is why this also catches my attention here. It also bothers me when I see both terraces right next to each other because I think a healthy distance should also be maintained, even if you get along perfectly; something would also be possible on both sides on the east and west side, especially since the 100% south side does not always have to be the best choice. Maybe also a second terrace. The large window fronts are my thing, although I would draw in the real furniture and see if they are supposed to stand like this in front of the window. You will both need shading there anyway because the furniture doesn't like direct sun for long either. Actually, I would find a "dark" flat roof over the terrace sensible for both sides. I would also skip the pantry with this sometimes seen closet passage. I also find the living room a bit cramped and despite the nice area not generous; sitting there with your back like that would not please me. Are those your real pieces of furniture then? I would provide a shower on the ground floor; with eventually 4 adults/teenagers, I would definitely find space for this to prevent upcoming trouble. At some point, one (I) doesn't want to share the bathroom with teenagers or have to experience pushing in a nice, newly built house. I cannot imagine the straight staircase; do you also have a drawing for that? All in all, a nice thing and a chic house. There is room to make necessary adjustments. In the upstairs bathroom, I would see if I can get a deeper shower without a door; a 60cm passage is enough in my opinion, whereas I would not necessarily need a double washbasin. In the master bathroom, there is either a door conflict or the frames will occasionally get wet. Therefore, maybe move the shower rather and also without a door; this is especially more comfortable at an older age when you don't have to pull off glass doors and bend over. The children's rooms upstairs are also generously windowed directly to the south; without my air conditioning, I definitely wouldn't want that. I also don't think children should look into the garden; the floor-length windows also interfere with use/furnishing.

The street and the entrances are on the uphill side and the living areas are in the basement, although I still lack a clear idea of the slope here. And (in-laws) are naturally a generation older than oneself – in this respect, I would consider two aspects: on the one hand, for you the question "who will follow them as your neighbors – will the children already be ready or will possibly unrelated tenants move into the house?" and on the other hand, for them the aspect that the daily change between living and sleeping levels is equally exhausting with age, regardless of whether the entrance level is the day or night level. Therefore, I would arrange as much as possible for the seniors on the entrance level and primarily spread the young family over both levels. Especially forget stairs, they are too space-consuming even with two flights, and a single flight is the "thorn in the side of the floor plan" which only works from about 12m house width (here referring to one half).

The draft itself is quite successful. Unfortunately, it is not clear in any way how the slope was taken into account. A site plan with elevation details is mandatory before starting to plan the house.

You need that anyway for the building application.

Thank you for all the feedback! The floor plan was made without exact heights, which are not yet available and are currently being requested. You can see in the picture in how the slope was taken into account in the design. We have also already considered a shower on the ground floor. We briefly sketched a straight staircase (completely different) which is difficult to implement... Does anyone have other ideas to enable the view from above onto the terrace? I even briefly thought about a 1x2 meter glass element in the floor of the upper floor. We also don’t like the terraces being directly next to each other 100%, however, this could be solved with an outdoor kitchen. Alternatively, the offset could be increased.

We have also already considered a shower on the ground floor.

On a hillside, I would rather speak of street and garden levels than upper and ground floors, otherwise that causes confusion.

You can see in the picture in how the hillside is taken into account in the design.

Unfortunately only very inadequately – otherwise I would not have said that I lack the imagination.

The floor plan was made without exact heights, these are not yet available, currently being requested.

Oh, that is dangerous (and also explains the lack of clarity created by the picture – from which fictitious height difference was assumed for the drawings?), when you "plan" without really knowing the basics. And even more so when you already create visualizations (which one can fall in love with and discuss for further modification). Did a house seller planner "draw" it like that? – I would immediately withdraw the mandate from a freelance architect if they planned without knowing the exact heights. The floor plans shown here are already detailed, moreover. A draft planning without a preliminary draft should never happen!

We once sketched a straight staircase (completely different) and it’s difficult to implement...

Hopefully that is enough to recognize for yourself what obstacles you are putting in the floor plan’s way.

Does anyone have other ideas to enable a view from above onto the terrace? I even briefly thought of a 1x2 meter glass element in the floor of the upper floor.

Why would one want to look from above onto the terrace?

We also don’t like that the terraces are directly next to each other 100%, but that could be regulated by an outdoor kitchen. Or one could increase the offset.

An outdoor kitchen is not cheap, and as a "visual barrier" even a decadent luxury. In this closeness you can again see why I mentioned the aspect of "non-family secondary users of the senior residential unit."