Ground floor approx. 100 sqm, upper floor expandable (planned bathroom, 2 children's rooms, 1 storage room)

According to the sketch by the OP, the roof pitch is 45°

Well, let's see what the OP can still manage with the stairs, then you really saved his bacon here, at least as far as the layout is concerned, in my opinion.

And I still consider the discussion about the attic pointless.

If the original poster comes in here with an obviously almost finished house with a "reserve attic" and neither upon seeing the real existing attic nor the plan recognizes that the desire for "3 generous (!!!) rooms, bathroom, and storage room" is not remotely achievable, then it will go just like with the ground floor planning: dozens of good and well-founded suggestions will be made, but the OP won't care a bit.

I feel like Kaho, I'm still in the head-shaking phase about how someone could have gone into building so naively and only then, when the house is practically finished, slowly think about how the upper floor should be. You just can't help anymore, sorry.
If one had given two to five thoughts beforehand to the still-to-be-built attic, one would at least have considered one or the other dormer or setback so that habitable rooms would be created up there. Or is the roof supposed to be torn open afterward for one or several dormers during the expansion? Now that's what I call forward-looking planning!
From how it looks in the picture (but it could also be that it’s just not visible there right now), no wiring has even been laid into the attic.

This is all so outrageous here that I don't know where to start or stop shaking my head.
Of course, you can also blame the general contractor, but if the OP was just as resistant to advice there as here, then at some point he must have thought: I'll build it for this fool the way he wants, his problem.
I would have the deepest understanding for that.

Wait!

I could also elegantly give in to a laughing fit!

oh, I have really set something in motion here

In advance as a refresher, so you don’t have to read the whole thread again, where I had already mentioned most of it

Children’s room on the ground floor: There will be a desk with a PC, it will never serve as a children’s room. It was just never important for us to change the designation in the plan

Bathroom: We decided on the following layout



We just don’t know yet whether we will do the boxed-in area at the sink or add a countertop later.

Kitchen: see children’s room. That’s how it actually is. Added to that is a table and corner bench. We will retrofit a shelf above the dishwasher. 400 € from the kitchen dealer was too much for a small shelf...



Doors: we reconsidered them yesterday and are with you – opening into the rooms.

Bedroom: I know the bedroom in real life. A 1.80 m bed would work, but very uncomfortable, so a 1.60 m bed, currently a 1.40 m bed is enough for us, so still an improvement over the current situation.

Terrace: a 5 m longer path won’t kill us, quite a bit fits on a tray. We were aware of that beforehand.

Upper floor: at first there were supposed to be two children’s rooms and one bathroom. From outside then came the comments regarding a small storage room and that we could also just move upstairs at first. We liked the ideas without thinking further about the implementation with the sloping ceilings. You have now put us on the spot regarding this. It will stay at three rooms (2 children’s rooms and 1 bathroom with bathtub) with luck another small room as a storage closet. That works too. I know that we didn’t do everything optimally and that has also sunk in.

Here a cross section



The attic is 2.4 m high, I still know that because we measured it. I think the upper floor has 2.60 m minus 10 cm floor construction and the ceiling cladding probably takes away another 5 cm.

The windows are currently estimated to be 5-8 cm below the beams. But that is because of the 24 cm roller shutter box at each window

A knee wall of 1 m doesn’t have to be. I think my children’s room had 70-75 cm (I will measure again this evening) and my bed fit right up against the wall and I could still sleep underneath it without problems as a teenager.

I basically like Hanse’s suggestion too.

The stairwell is 3.5 m long and 1.1 m wide.

: I find criticism okay, we certainly also offer a good target. You’re slowly going beyond it. After the first criticism there don’t need to be two more posts that beat the same drum. Enjoy your project, which has gone great so far, but please don’t spoil our project because it’s not 100% thought out. We are now trying to make the best of the situation. Luckily you don’t have to move in

Have I forgotten any information?

oh, I really set something in motion there

Yes.

Did I forget any info?

Yes.
I like the latest draft for the upper floor best. But it only works if you optimize the staircase. So what else can you do with the staircase?
Where exactly is the hole? Can it be moved or shifted? How long is the staircase exactly? Has it already been ordered? What is the floor-to-ceiling height on the ground floor (i.e., what height must the staircase overcome)? Is the roof pitch really 45°? Where exactly are the windows on the upper floor? What empty conduits have you already planned or installed?

The staircase is marked in the plan as 15x18.57 = 2.77m. What distance do you actually have from the ground floor finished floor level to the upper floor finished floor level? The 2.77m is, according to the section, from the raw concrete floor to the underside of the beam.