Floor plan design single-family house with around 150 m²

But the contract is already signed with the general contractor including the architect.

Contract for what: generally to build with company XY, then the attitude "outside there are only pitchers" does not apply. Explicitly for the design 08/15: then it is something else, then there will be no more than the 08/15 version D.

Contract for what: generally to build with company XY, then the attitude "outside there are only pots" doesn't hold.

Yes, that's usually how it is. The contract is with the general contractor including architectural services for a house with exterior dimensions xyz. The floor plan is then a design, which is quickly scribbled and then goodbye.

Contract exists with the general contractor including architectural services for a house with exterior dimensions xyz. The floor plan is then a draft, which is quickly scribbled and goodbye.

Unfortunately, that's how it is :-(

About the floor plan: Apart from the zigzag walls, - how wide are your doors on the upper floor? - What is the square meter size of the children's rooms for you now? - Have you ever tried to draw the 2m line for the storage room? - The zigzag for the fireplace would only make sense if you actually had an offer with measurements for the fireplace.

The floor plan is then a draft that is quickly scribbled down and that's it.

At least you have already contributed suggestions so that the OP is not left empty-handed. If necessary, one then makes a selection from these, from which the "architect" creates a submission plan. He should be able to draw in the winding of your stairs.

Regarding the floor plan:
Apart from the jagged walls,
- how wide are your doors on the upper floor?

On the upper floor, all doors are 88.5 cm clear passage width, except for the storage room with 76 cm clear passage width.
I think that’s okay.

- What is the square meter size of the children's rooms now for you?

The children's rooms are about 14 m² (the left one possibly only 13.5, but it fits)

- Have you tried to draw the 2m line in the storage room?

The storage room is about 2.5 m deep, the 2m line will be roughly at 1m. Tall cabinets will be placed against the interior wall, smaller sideboards etc. against the exterior wall.

- The notch for the chimney would only make sense if there was actually already an offer with measurements for the chimney.

I have already chosen the chimney. The exact centimeters don’t matter because a drywall shelf, which is adjustable in width, will be placed to the right of the chimney (also into the recess). Of course, I can ask the chimney builder again how much he roughly plans. But I think you can count on about 55 cm width for the chimney + 90 cm for the stove + 55 cm for the drywall shelf).