House construction financing: okay or better to make cuts?

and how others calculate like that

I just think your calculation is too meager.

Land: €100,000 House ready to move in: €500,000 Ancillary costs: €60,000 Broker and tax: approx. €10,000 total: approx. €670,000

You ask whether maybe the carport should be left out, although exterior facilities are not listed at all. There is also no real ready to move in: even if a construction company still does the walls and floor coverings for you (which is very rare nowadays), costs will still come up, e.g. kitchen and lamps. I don't know any builder who got through the foundation slab without additional costs. You include a good value for ancillary costs, which are probably building ancillary costs. The catalog price is very often quite bare... you can do it that way, but a small additional item for upgrades during construction and a foundation buffer should definitely appear in the calculation. For me this total of 670,000 is off. It is already a lot, but you have to assume that it is not the final price. I can only advise you to explicitly list everything once, e.g. notary, driveway and terrace, splash protection, rainwater infiltration, electrician, etc., so that you yourself know what it will amount to.