Construction Costs for Single-Family House - Is Our Cost Plan Realistic?

A general contractor/general planner does not itemize individual items

So individual offers. Then I would request a second offer each time. And if necessary, additionally discuss individual offers here in the forum (one thread per offer/trade, indication of price and specification of the underlying service). It is much easier to compare and evaluate in this case than with a complete offer from a general contractor/general planner. Incidentally, I wonder whether an architect or construction supervisor is involved. Who obtained the offers? Based on which scope of services? A piece of shell construction? That only works with a design, or am I mistaken? And then the architect/construction supervisor should also be able to say something about the costs.

Good point to put the individual items up for separate discussion, but they should rather serve to give us an overview of the expected total costs, hence also rounded. Of course, a draft of our ideas already exists, i.e. planned. Floor plan. As for the heating, that is naturally only a rough estimate. Thanks for the hint about geothermal energy, we will look into that.