Unpredictability of construction costs - Will the project fail because of it?

One cannot assess your situation from a distance. However, it sounds as if you are, with the considerations and real preparations (including the sale of the condominium), so far along that, from my humble perspective, scrapping all plans would be the significantly more expensive and riskier option. As I said, no advice but a thought I would have now: If in doubt, refinance? Consider parents and grandparents for private loans? The market and conditions will not get better. There are also examples here in the forum of surprisingly stable prices despite the difficult situation. Hang in there, keep fighting, talk to the contractors.

We have not spoken to the bank yet. As I said, a property change would probably be possible. Otherwise, the regular prepayment penalties apply.

I assume that you are planning a house with 2 floors, totaling 125 sqm. With a basement you then have 180 sqm. Or something similar. Include the basement as living space and do without the attic.

You signed a contract with a fixed price in March. What do you still have to sign now?

Then say it: I asked indirectly: how is it now? How much are you really planning? 180sqm? 220sqm? Are you aware of that? Is there any way to do without one level? Then discuss it with the general contractor... make it smaller for yourselves, and then go back to the bank.

We are now planning 125 sqm of living space + basement. In terms of external dimensions, it's hardly possible to reduce further, as it's already only 8.10 x 10.50. If anything, one really would have to omit a level.