Is construction financing possible like this? Or where is the mistake?

I do think that under the given conditions a financing offer can be obtained. The rule of thumb has always been: at least 20% equity.

Maybe the cost plan was also too inaccurate? I presented the bank with a nice, relatively detailed cost plan and they were very pleased. The advisor said that most people wouldn’t do something like that or would underestimate the costs. Then the bank naturally has to react somewhat more cautiously.