Design for a single-family house with 160-180 sqm - suggestions for improvement?

Then the other rooms on the upper floor become too small. And how is it with the cold basement with open stairs?

Are you building a stairwell around the staircase from basement to ground floor? Looks open according to the plan.

Either a stairwell for the basement stairs, there the stairs to the upper floor also go above.

The upper floor will not be too small. You have potential in the hallway and bedroom.

not to get hung up too much. Here you have to skillfully zone: different wall designs in both areas, possibly different lighting concepts.

I would consider that if it is an existing building or if walls are being torn down in an old building to enlarge it. Or if a guest room has to be made a bit narrower. When building new, you make sure that something like this is almost coherent and you don’t have to work with effects.

But there is absolutely no need to think further about this because the is completely inconsistent.

You are right about that too. However, one has to emphatically point out the obvious inconsistencies to the OP here

What exactly is completely inconsistent?

And here we go: the bedroom has a full south and west side. You can’t get more heat into a room than that. In the end, every window here will have to be darkened when the sun is out.
I advise orienting the bedroom to the east.
And when you do that, other things will also change upstairs.

The hallway looks awkward not only because of the two opposing staircases, it's also larger in terms of space than the living room. Is that coherent?

In the basement, I would not open the staircase into a cold room but rather give it a small hallway at the bottom and a wall that limits the staircase.

I would insulate the basement rooms

The technology has to be done anyway, and then you can do the rest as well. This will be your personal entrance. Why not make it warm and nice. An open staircase going up is also cozier than a dark corridor with a door at the top that is hard to open with full hands.

I would plan a platform staircase against the upper wall.

The problem with the bedroom is that the neighbor is to the east; to the west there is no way to be built on - therefore the orientation in that direction. You can actually keep the bedroom shaded during the day without any problems, right? You usually spend time there rather in the evening or morning?

What do you mean by the hallway in the basement? What would that look like? Does the utility room have to be insulated? Do you usually insulate in the basement from the outside or inside or both?

The living room should actually be 40sqm; hopefully the hallway isn’t that big.

You generate heat in the technical room, your water must be protected from frost. Therefore, technical rooms are usually within the thermal envelope.

Despite the sloping site, we have a basement and only one side is underground. The wall is insulated on both sides because of condensation.

Create a nice comfortable entrance without the feeling of crawling out of the basement. Different staircase Remodeling of the upper floors

Your hallway plus the 2 staircases need at least 20 sqm

What happens to the bathroom shown, which is no longer a bathroom?