12x9.6m 2 full floors, basement, attic, 4 children's rooms

I can't help it *g*

When you work from home, it ideally needs its own room, I agree. Especially with so many offspring
If it were only about making phone calls undisturbed, I would spontaneously suggest the parents' bedroom.

That your wife would also like a room for herself, I can basically understand, but you really have to ask how essential that currently is. With 4 kids, eventually the first children's room will become free, then she can take that, and until then I would at least aim for a temporary solution for this. That can be a corner in the parents' bedroom (which is usually unused during the day) or a room in the basement. Just without daylight, but since she won't be there for hours a day like with working from home, that would be a practical interim solution for me (that's where we have to make compromises, which are not nice but bearable)

Hi Climbee, we are not that far apart, but she wants it, and if everyone has the same size, it's fair. I also don't find the space usage on the upper floor bothersome. The children's rooms are all almost the same size, the bathroom is fine for the crew, and that's basically all you need. For my work, sometimes I am at home all week, sometimes on the road all week. Without my own space, it definitely doesn't work. I just added the kitchen with an island and rearranged the sofas. The latter could also be placed side by side perfectly if you want to have a movie night.

The air exchange in the office is ensured by the ventilation system.

It would have to exchange air practically twice as fast in a room half the size.

You are right, it is pointless to talk to you about costs without having the offers. Good luck with your [Grundriss].
I can already imagine your next posts like "Isn't the offer too expensive" :-p

I just added a kitchen with an island and rearranged the sofas. The latter could also be placed nicely side by side if you want to have a movie night.

So I think the kitchen island is oversized and the sofa rearrangement didn’t really make a big difference, if you ask me.

Here is another version where I took up your entrance from the street again. The goal was a guest WC and a bedroom not directly facing the street.

Advantages: Spacious entrance area, you can even park a stroller there. All rooms accommodated. Bedroom not on the street side, guest WC, children’s room almost unchanged in size.

Disadvantages: Window behind the bed in the bedroom, is the common room still sufficient for 6 people? Placement of the garage questionable.

Three notes from me:
1. With the hammer fit of 10m GH, you should pay attention to the reference point. Your planning only works if the street is perfectly level or slopes upwards to the south.
2. Contradiction:

Anzahl Stellplatz nur vor der Garage geplant

Ein zweiter Punkt ist, dass wir auf der einen Seite die Garage und auf der anderen Seite unseren Wohnwagen haben.

With the latter statement, the floor area ratio could become tight.
3. The area dimensioned with 3.00m at the top of the plan is presumably the terrace. This is located outside the buildable area. My advice for the later building submissions: better not to show the terrace. Then point 2 will also be resolved.