Follow-up financing experiences?

I see the problem almost only with the new house. It is financed at 0.9 percent. If the interest rate goes up to 4 percent, for example, nothing will be left, and the building also gets older, i.e. maintenance costs.

You don’t need to worry. Today, nothing would be left at 4 percent. But in the next 10 years, you will have already made about 210,000 euros surplus with the house, with which you can pay off the loan for your single-family house. In 10 years, you will get at least 20 percent more rent for the multi-family house, so even at 4 percent interest, there would still be something left to slowly pay off the loan. The most unwise thing would be to repay this super cheap loan (0.9 percent interest) too early. Only the tax office would be happy about that.