Floor plan of a single-family house, 2 floors with basement, approx. 190 m², plot approx. 440 m²

enclosed space including basement a fixed price of €346,000

Basement? :eek:
That is just over €1,800/sqm ... corresponds to the construction price of 2014 / 2015.

which according to the construction service description already has a high standard

That contradicts itself. Sorry.

Here are a few more pictures of the property. All the concrete foundations and slabs will be removed. Although we will probably leave the slabs and such as a construction site setup area for now. The fence and the wall along the street will be removed beforehand, so the property will be level with the street along its entire length. It’s hard to see, but about in the middle, the property slopes down more steeply along the street. Further up, the incline is not as steep. The wall along the street is 90 cm high at the lower corner of the property.

: well, that's the price he calculated for us. It's good for us if he's apparently calculating with prices from 2015 :D According to the construction service description, all basement rooms have a raw construction dimension of 2.635m, with flush-mounted electrical wiring, underfloor heating, etc. Of course, if we now want larger windows, ventilation systems, and so on due to basement living requirements, that naturally comes with an additional cost. But that will probably be around 10-20k and not 100k.

For 190m², from my perspective, the second bathroom is definitely missing (for 4 people). But even otherwise, I do not find the layout particularly smart. The utility room is hardly usable as a corridor: where should one hang laundry there (only one point)? Those are just two points. The professionals will surely find more here. If one is to evaluate the floor plan, above all measurements are missing (e.g., how wide is the "Garderobe"/vestibule? I have the feeling that a sensible cloakroom for 4 people cannot be implemented there.

Further regarding the budget: ypg hinted at it, I say it clearly: forget it! 1800€/m² is not feasible - in any standard. The attitude here is wrong from my perspective: (in essence) "It's good for us if it is so inexpensive/cheap (watch out)" No, it is not, because the price is a utopia. Reality will catch up with you. From my humble perspective, you should not plan under 2500€/m² + additional construction costs. The general contractor is NOW calculating a nice 1800€/m² for you (and that "turnkey" - again watch out!), but the real numbers are yet to come.

You do not want a fundamental discussion about architect/general contractor/consulting architect/building owner, but one thing is certain: to plan a usable floor plan requires a professional. Whether they now come from the general contractor or are an independent architect does not matter.

Bathrooms do not have to be on top of each other, but bathroom above the dining area should be avoided. Children preferably in the south rather than in the north. 3.50 living room width, many narrow rooms, entrance situation multiple times around the corner not inviting. With stairs having a 2 meter stairwell width, you get 75 cm stair width. Many niches poorly usable. Also the corridor not. Windows in the living area are quite narrow.

well, that is the price he calculated for us.

I assume he didn't calculate, he estimated?