Financing problems

I suspect your construction cost breakdown is not plausible or contains errors. The 100% financing should not really be the issue. 285k€ feels too low intuitively without knowing the details. There will still be some on-site costs, and I do not mean painters and flooring. The desired security of 75k€ from [Sparkasse] roughly indicates your construction cost gap to you.

Only sorry who advised you during the construction? You don’t make a notary appointment without having financing, do you?! How many € are now available as equity?

100% financing is not available everywhere
50,000 without proper offers modest equity for the income and without rent
The two banks are in a tight spot there

Try it through a broker. He has more banks in his portfolio.

Amazing that at least the Sparkasse does not suggest alternative financing options, such as a 400k loan with 0€ equity entry.

You need a broker who takes the time to go through the basics with you in a personal conversation and then creates a new financing concept with you.

Are there Interhyp, Dr. Klein or similar nearby?

Sorry the bad question just shows that you are acting hastily. Didn't the general contractor recommend a craftsman? 25k for additional construction costs? That also includes connections, outdoor facilities, earthworks? Bold. It says nothing if someone doesn't have to provide additional financing. Maybe they borrowed more. Or who is the statement from?

There are solid reasons why a bank rejects someone. I know people who have borrowed more money and did so without significant equity (10k). Are there commissions in the salary or any irregularities?