Change upper floor layout partition

The staircase comes from the ground floor, the architect probably drew it incorrectly, but I didn't notice.

This long narrow box is not everyone's thing. Do you really want that?

Well, I might still imagine something like this:

[ATTACH alt="Dachgeschossumbau.jpg"]85281[/ATTACH]
But it needs a new window in the children's room. Or you swap the bathroom with the child's room, depending on where the pipes are.
I also wouldn't be sure if the thick wall doesn't play some role in the statics.
But you want a smaller bedroom, so here's another option:

[ATTACH alt="aufteilung-grundriss-og-aendern-660235-1.jpg"]85282[/ATTACH]
It all remains guesswork as long as it is not clear exactly where the pipes are and whether a roof window could still be installed.

Oh, so it could also come out at the top of the plan and not opposite the bathroom?
I suppose you or you all know better how much space you need for bed and wardrobe, whether two meters are enough or it has to be four meters. There are also big and small must-haves for the office... if nothing is load-bearing anyway, then just do it the way you want. We don't know anything about you... and ultimately, based on your other posts: you will probably implement it yourself, so plan it so that you can do it.
There aren’t that many options anyway.