Buy a single-family house - mortgage a condominium unit and sell it later?

And then please calculate how long you will need if you do not sell the apartment or what installment you can afford.

How high is it actually? I don’t get a large surplus after taxes, non-allocable operating costs, and maintenance reserves. And then stress because of that?

Yes, that's why the option "apartment sale" was the A variant. However, we are still considering how best to finance the new construction project...

Nothing comes from nothing. Even if the apartment is paid off, you pay taxes on additional income. Costs can be deducted and the surplus pays off your own house proportionally. Of course, it is a risk. But certainly not a HIGH risk.

I'm not concerned about the risk either. It's about the fact that the OP has a cash surplus of just a few hundred euros per month, but in return has a payment of well over 1,000€. You can do that with the right income - but then this question wouldn't be asked here in the forum.
And the surplus from the sale quickly pays off the own house proportionally right from the start ;)

To make it clear: in the long run, there would be more money available financially than without the apartment.

there would be more money available financially than without the apartment