realistic cost planning

What BeHaElJa says is correct. Get the catalogs from the providers and google the prices. That is a reference point... and then I would rather add 10% than 10,000€ on the price. Just sockets, light switches, network sockets, other doors, underfloor heating, that quickly adds up to 15,000€ and more

... We currently have an offer of around €250,000 for approximately 160 sqm in solid construction Ytong. However, this is purely the standard, without special requests such as a chimney, controlled residential ventilation, etc.

Depending on where you come from, it could be around €230,000, but without major extras.