Thank you again for all the numerous responses.
There would be a good retirement income available and debt-free real estate as collateral.
However, we are still at the beginning of our search / our project and have simply done some googling and seen prices in the range of 250,000 euros to 300,000 on general info homepages.
400,000 is of course a different ballpark.
As mentioned, 150,000 is available as equity and taking on another 250,000 in retirement is, I think, not sensible, of course regardless of whether one would get a loan at all.
It is difficult, but for the moment or rather probably forever, we are stepping back from the idea of putting another house in the garden (our building plot)...