How does the additional payment obligation actually work?
I’m not exactly sure what you mean. As I know it, you first have to use up your equity for the purchase, construction, etc. So only the
specified part of the equity that you voluntarily want to contribute, not the reserve kept in the background. After that, there is the money from the bank. Exactly as much as agreed upon. I wouldn’t know why you would then have to add more equity to the bank. You only have to add equity if you spend more money than planned, but then the money goes to those from whom you bought: forgotten cost points, price increases, special requests, etc.
When does the bank come knocking if the real estate market crashes?
No one comes knocking. As soon as you have signed a contract, the contract stands unchanged. Whether the market crashes... doesn’t matter. You get the agreed money and you pay the agreed amount. The same applies if prices skyrocket. You only get the agreed amount. Just a contract. The bank only comes knocking if you no longer pay your monthly installment. How you pay it is your problem: salary or reserves in case of unemployment.