Amount of the monthly installment

I think it is not productive to approach the OP like this here...
One should rather give her tips on how to make the best of the situation:
- have the contract checked
- go to a financial advisor
- definitely not build bigger

The income is already sufficient, no equity is of course bad but also feasible.

Well, then I don't know how the others here in the circle of friends, acquaintances, and family do it.

If you include it for security and you yourself go bankrupt, the parents' house, if your resale price is not sufficient, will also be lost

Do you (almost) all seriously believe that an average earner can never afford a little house? Is it only possible with a monthly net income of at least 3k per person and preferably equity of at least 50k? Then I don’t know how the others here in the circle of friends, acquaintances, and family do it.

Regarding the security, I agree with Wilhelm. If you include it for security and you yourself go bankrupt, the parents' house, if your resale price is not sufficient, should also be gone

Whether they really could, time will tell. You can't use the builders from 10 years ago as a comparison because price increases since then have been significantly higher than income increases. 50k as equity is really something like the lower limit to manage anything at all. By the way, thanks to your parents' property, you have significantly more.



That is true, by the way. If you are stuck with the mortgage, in case of doubt, you don't decide what happens to the family home, but the bank does. Sorry to say so.

My parents have a lifelong right of residence / right to live there.

Sure, it would be unfortunate if we went bankrupt, but I don't expect that... if I factor something like that in, I wouldn't even start the topic of building a house. ...