Floor plan design single-family house approx. 170 m² + attic as a modern clinker house

Where should the planned 3rd child sleep

The budget will be more than tight. 170 sqm + brickwork + garage + high standard + equipment is likely to be very tight. You should be aware of that, not that the 550k is your absolute pain threshold.

I thought so too. If I extrapolate from our project, there are 50 to 100 K missing.

Where is the planned 3rd child supposed to sleep

A third children's room is to be built in the attic.

A third children's room is to be created in the attic.

From current experience, I have to say: I would no longer have an office on the ground floor, especially in the living area. When working from home while the kids are at home, I retreat to the upstairs guest room, which has now become my office. At least until the attic is finished.

In the ground floor office, one of the children constantly comes in, knocks on the door, and wants you to come with them to pee or to warm up the milk, etc.

It's all a matter of upbringing, I would say. ;) However, I also find access to the office from the WZ rather unfortunate.