ᐅ Single-family house design, 250 m², 2 full stories with a hip roof

Where is your architect? Do you want to build 250m² (2,690 sq ft) without approval? According to German law, you have to hire an architect to draw up the plans anyway. Why don’t you ask them first? They can explain and calculate many details that would otherwise fill countless pointless pages here.
With the results, we can then perfect the finer points.

Katja, don’t get worked up here!

I can only keep referring to the price estimate (allegedly backed up by quotes), and when I see the bathroom, I can’t help but smile.

You just have to think about that statement once more.
We all know where this is going.

...Price estimate: 360k Euro (including kitchen, bathroom, floors, electrical, heating... turnkey)...

Thank you for the occasionally constructive feedback, and I recommend a bit more calmness.

The land and exterior work are not included.

As mentioned, many of the offers come from acquaintances and can be considered affordable.

Some tasks will also be carried out as self-performed work.

I also find it questionable why someone would need so much space at that age. But to each their own, or depending on what is possible. However, with that budget, you won’t be able to achieve it. ...

Are you seriously going into detailed planning with this very poor basement floor plan? It would be really disappointing if such an unusable house is built on this huge area.