Floor plan design for a single-family house approximately 125 sqm, 2 full floors

How did you calculate the 135-140sqm? When I add up the individual rooms, I get about 128sqm without considering the staircase.

And with the staircase considered, around 133, right? If that’s okay, then that’s fine.

here's my idea, same external dimensions and assuming that the statics can be managed. (I also left out the windows)

kitchen and dining area can have direct access to the terrace.
Space for a wardrobe and the utility room has also grown (no idea if that's enough now)
there is still space in the WC for towels and cleaning supplies).
bathroom above WC. guest room and office combined. child has S/W orientation though.

EDIT: OOPS the guest bed has shifted a bit

Nice straightforward idea - I would still shuffle things around up top

hm well, I don't find the orientation on the ground floor that great there. don't know why "everyone" always wants their living room in the SW. I think NW is much better suited, and then the kitchen is in the SE and not in the NE and so you still have sun in the kitchen at midday/afternoon (you prepare food several times a day, I sit on my sofa rather in the evening )

I totally agree

Say again why guest rooms and office are not allowed to be combined.

Hopefully, you have already noticed yourself that the kitchen setup is unattractive. Likewise, people very rarely like having the head of the bed next to the door in the bedroom.

Say again why the guest room and office are not allowed to be combined.

Where does it say that?
A house doesn't work if too many rooms are squeezed into a small number of square meters. I see too few storage options for cupboards and such in your original idea.
If I were you, I would untangle the knot in your head and look at and evaluate the house matter neutrally.

Where does it say that?

Yes, exactly nowhere! But he keeps planning it like that, although in my opinion the space would be more useful elsewhere and you had already suggested that.

Who says and why?
In the typical tent-roof villa, I rather see drywall partitions on the upper floor (or at this point aerated concrete, if you will, in any case no problem).

I am not an expert but I have been told that the load-bearing walls in the case of a solid concrete ceiling on the upper floor are best positioned directly above each other. Whether this also applies to our house, I have to ask a general contractor/structural engineer.

here’s my idea, same exterior dimensions and assuming the structural issues can be resolved. (I also left out the windows)

Kitchen and dining area can have direct access to the terrace.
There is space for a cloakroom and the utility room has also grown (no idea if that is enough now)
there is still space for towels and cleaning supplies in the WC).
Bathroom above the WC. Guest room and office combined. Child’s room faces S/W though.

Thanks for your design. The idea of placing the staircase in the east looks good.
What we do not like so much, however, is the open kitchen. We would like a somewhat more closed kitchen so that it is not completely visible from the sofa.

How about using the Flair 130 as a basis ...

The Flair 130 has an even larger footprint (10.10 m x 9.10 m).