Is it common practice with the buffer and does the bank always approve 10% on top for safety, or how is that sold?
As far as I know: You have to deduct the buffer from your equity beforehand. So
+ Production costs according to a serious cost estimate or developer contract
+ Additional costs (notary, property transfer tax, realtor)
= subtotal
- equity that you want/can contribute
= capital requirement
The bank decides yes or no and states the interest rate (which depends, among other things, on the ratio of capital requirement to production costs).
Where is the buffer? Without the bank, just for yourself, you do the following calculation:
+ subtotal from above
+ everything else (kitchen, lamps, moving truck, KfW fees, expert, expenses, construction period interest, this and that, something else, insurance, ...)
= total capital requirement for the builder without contingencies (UV)
+ contingencies (UV) (for example, a dinosaur skeleton is found during groundworks. Or a glacial erratic. The only available electrician becomes incurably ill. The developer goes bankrupt. Or something really unforeseen happens.)
= total capital requirement including contingencies
How many euros you put in contingencies depends. On the contract situation, on the circumstances.
Whether the bank gives you that depends on whether the "capital requirement" is greater or less than "production costs." And on your creditworthiness.