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

And considering the stairs, about 133, right? If that's okay, then fine.

Almost (approx. 131sqm). Whether that still fits I will find out once we have had the first talks with several general contractors.

Tell me again why the guest room and office may not be combined.

Hopefully, you have already noticed that the kitchen setup is unattractive. Likewise, it is very unpleasant to sleep with the head of the bed next to the bedroom door.

I would prefer a separate study. Then it also doesn’t have to be (always) tidied up when visitors come. However, I am also aware that one has to make this or that compromise. So far, these are just ideas and nothing is set in stone.

The kitchen is not planned yet. In principle, we like a small cooking island. Of course, the question remains whether this can be sensibly implemented in a tight space.

I am not an expert but I was told that the load-bearing walls in the case of a solid concrete ceiling on the upper floor are ideally aligned.

Yes. However, I do not see any compelling reason here for a concrete ceiling on the upper floor, and thus not for its load-bearing walls, so all the non-load-bearing walls of the upper floor could then be placed more flexibly.

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

The suggestion probably referred more to this house model itself rather than necessarily to its dimensions.

I think a home office doesn’t have to be tidy at all at least we don’t really do that (it’s just for working ) but I worked on a new version. The external dimensions are 8.01m x 10.65m, no idea if that fits with the building envelope. I can also gladly provide more info on the individual room dimensions. Unfortunately, the program can’t measure autonomously. Walls outside 40cm inside 20cm, stairs 200x220cm

Ground floor: Viewing axis now no longer so strongly towards the kitchen (or still too "open"?). Second entrance if desired e.g. for the rotary dryer, alternatively space for a cabinet.

Upper floor: Separate guest room, for that the bedroom is somewhat smaller and so is the children's room

If the office = guest room, it simply has to be tidied up when guests come. I think that's what the OP means.

Then think about how often that is the case and whether it wouldn't be better to tidy up the office three or four times a year for visits instead of having two bays that don't deserve the designation "room."

I see it exactly like Climbee, better to tidy up 3-4 times a year
that would be my favorite

I see it exactly like Climbee, better to tidy up 3-4 times a year
that would be my favorite

I really like the bathroom