Single-family house with flat roof and 367 sqm living area

But not with someone who uses her bathroom, she is right about that.

The bathroom belongs to the guest and the Au pair. She is allowed to use it. Where does it say that an Au pair needs a separate bathroom?

Imagine a foster child of college age who is helpful around the house (unfortunately without that rubbing off on their own kids).

I like your very fitting description. :cool:

The bathroom belongs to the guest and the au pair. She is allowed to use it. Where does it say that an au pair needs their own bathroom?

hmm... an au pair uses and lives in the spaces where the family does. The objection from is not unfounded: why is the room in the basement, but the children’s rooms are upstairs? The OP already mentioned a granny flat, so there is an all-round zone for visitors-whatever on the floor area. If I stay somewhere overnight for one night because they don’t want me to endure the travel distance, I need a toilet and a sink for brushing my teeth for 10 minutes. I don’t need long bathing and showering times, I can do that at home. And even if I were given the space, I wouldn’t use it because the cleaning effort afterwards doesn’t justify it. The parents-in-law, who stay a few days longer, are about my age and should be glad to be able to use a shower at all.... I see a split-level here or at least bedrooms with entrance and garages on the ground floor. In the basement, then utility room, freezer, fitness room, and of course the all-purpose room facing south. Fitness and utility room can, for example, benefit from a light well in the front west. Upstairs then work and master bedroom. Or guest. Something like that. In my opinion, the desired openness is not achieved by a covered loggia in the all-purpose room, but by an almost barrier-free openness to the garden.

The bathroom belongs to the guest and the Au pair. She is allowed to use it. Where does it say that an Au pair needs her own bathroom?

I am not of the opinion that the Au pair needs her own bathroom or assumed that she shares the bathrooms with the rest of the family. What bothers me is just the thought that male overnight guests, relatively far away from the rest of the family, use the (among other things) bathroom of the Au pair at night, and she is lying right next to it in bed. But it may be that only I find that inappropriate and for various reasons I do not want to go into further detail. But as I said, maybe my point of view is totally inappropriate.

He probably didn't expect so many suggestions for improvement. It's not that easy to address all of them.
But when even simple follow-up questions remain unanswered, I have some suspicions about why the thread was started.

or spends his Sunday outside instead of in front of the PC :p

...
That was also my first thought: that the architect failed to tell the clients that they should put their Pinterest wish list in an envelope and seal it. ...

I like the nicely phrased descriptions by

I only know one AuPair (related to me) and the family that employed her (for 20 years). Based on this, I will make some assumptions: rather well-off families, usually both working, hire AuPairs. Also to raise children bilingually, while passing on knowledge (own language skills -> AuPair). By the way, there are also male ones! I have never heard of clients building specifically "AuPair-friendly" houses, and that is due to the limited period / the appropriate age of the children. Having an AuPair is, in addition to the gain in comfort, a status symbol (my perception and without wanting to disparage). The planned AuPair area will simply be a guest area (or something else) in some years.

I have experienced AuPairs more as "the older sister" or the "very good friend" of the family who devotedly takes care of the younger ones. Integration into the family is sensibly deep. A separate room is obvious (AuPair is entitled to that), a guest bathroom separate for the AuPair is presumably usually more common in families like the ones mentioned above (in DE). Joint vacations, shopping, eating... whatever... is also not unusual. But so is the AuPair's privacy. But one question to the OP would also be whether he/she has already had experience with an AuPair? AuPairs have existed for quite a long time now... Many agents meanwhile impose high demands on the host family for various reasons. These must be met.

My husband is watching sports, so I'll just quickly put together a SKETCH before dinner – or rather 3D views... if it were my house, if I had all the requirements like the OP
on the ground floor... about 15 x 16, something like that, front and rear projections variable, since the staircase is almost centrally located. Windows are just examples in the larger rooms and measure 2.40 meters in width
Inside everything fits too
- Double garage with bicycle space check, utility room check
- 2 children's rooms, guest/au pair area, study, library check
- in the basement, a family room, fitness, utility room, freezer, wine cellar and the like
- on the upper floor, parents' area with workspace
Check, check, check
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- I forgot the piano, but that would fit somewhere too, since much is variable – at least without architectural knowledge :D
If I defined the outer walls correctly in their thickness, I would come out to around 360 sqm. With my 10 cm, I'm roughly at 380 sqm including the garage.