Building a single-family house with a basement - Is the financing calculation too risky?

I think I expressed myself incorrectly there, we only need to finance 500,000 euros of the 700,000 euros in costs, 200,000 euros is equity. We have also planned another 150,000 euros for the kitchen and outdoor facilities and this amount is also available. The 3,000 euros naturally also include 250 euros in reserves for the house, life insurance, etc. included...

The house costs 700,000 because the basement already costs 110,000 due to the steep slope.
Then the house costs a good 600,000 for 200 sqm.
Not bad.

However, we have high monthly fixed costs due to insurance and savings contracts and of course a high standard of living, so we spend 3,000 euros per month on that. Is it unrealistic to plan like this?

Regarding your question:
Yes, I find it a bit exaggerated.
Salary is great, equity is also good, concessions are made due to the high standard of living, then the 600,000€ house project should be reviewed. It is certainly an architect’s house (not a general contractor) and thus, experience shows, about 20% higher costs creep in.
A 50,000€ kitchen certainly also adorns a house that is 20 sqm smaller or the facade is not quite as elaborate, or whatever else makes the house expensive.

But it is a construction company, not an architect. The costs are so high because of the fittings. In the basement, we are planning a living space for visitors, an office, and later for the children when they grow up and possibly want their own area due to the slope. That is why (wegen dem Wohnraumausbau) the basement costs 110,000 euros. We want solid doors, wooden floorboards, windows with burglar protection, fly screens, electric blinds, and internal muntins. Wooden floorboards, etc.

But it is a construction company, not an architect. The costs are so high because of the equipment. In the basement, we are planning a living space for visitors, an office, and later for the children when they grow up and possibly want their own area due to the slope. That is why (because of the living space expansion) the basement costs 110,000 euros. We want solid doors, wooden floorboards, windows with burglary protection, fly screens, electric blinds, and interior muntins. Wooden floorboards, etc.

If we orient ourselves again with 2k€ per sqm, the budget will probably not be sufficient or not sufficient for everything completely. Also, the 2k€ per sqm mark does not seem to fit the circumstances.

The offer is available to us at the amount from the contractor. 2000 Euro x 200 sqm would only be 400,000 Euro, we are at about 600,000 Euro...

That's fine as it is. I would, however, plan a bit more leanly in your place so you can sleep well and not possibly have to restrict the 'high standard of living' after building the house. 100,000 for the outdoor area is quite a sum; it can be done more cheaply. Also, a kitchen for 30k is the upscale version. Give yourself a bit more breathing room; something unforeseeable always comes up anyway. Overall, though, an excellent starting position for building the house, congratulations!