Thank you very much for the suggestions.
When a bank grants a construction loan, they only lend the money for actual construction costs, right?
What happens if the 15,000 cash after the purchase is not sufficient? Will I then get money from the bank?
If the 15,000€ cash is not enough, you will also get what you need additionally. The decisive factor for an appraisal is always ALL costs, including incidental costs. In the overall consideration, it is decisive whether you come to a financing need below or above 100%.
Example: You have a plot of land worth 100,000€. 0€ cash. The house costs 300,000€ and incidental costs 50,000€.
The 50,000€ for incidental costs you will also get without problems from the bank in this case, because you bring Römer as the equivalent in the form of the land. So you will get a total of 350,000€, probably also without exceeding a loan-to-value ratio over 100%.