Realistic cost estimate: Single-family house with unfavorable development location

but of course it is our contractors who have to judge

They build everything for you that you sign. Even a building authority can overlook bottlenecks, and dumb spots (i.e. unattractive but not non-compliant planning) even more so. Then "post approval stamp" is improvised, for example, the consequences are dealt with in the form of a shower changed to 80 cm width or something similar. No, builder means: person responsible. You can’t just rely on professionals.

Floor height is 2.75m shell construction.

15*18.70 = 280.5 cm floor height according to the stairs.

15*18.70 = 280.5 cm floor height according to the stairs.

Which corresponds to a clear room height of a modest 2.4m to 2.45m.

Which corresponds to a clear room height of a modest 2.4m to 2.45m.

As long as none of them are 2.35m tall, a room height of 2.45m should not be a problem.
On the contrary, the less volume, the less heating is required.

the less volume, the less heating is needed

The post-war built apartments owe their ceiling heights to this Flat Earth simplistic calculation.

Enlighten me about the advantages of a 2.8-3.4m high living room.