I think it's good that you registered here. But unfortunately, the matter can hardly be taken seriously.
What does the dream house for 280K look like and what about the plot? What all is included with the place and how big is it? How much buffer are you planning, how much additional costs?
Some more reading on this:
https://www.hausbau-forum.de/threads/liste-der-anfallenden-Baunebenkosten-bauseits-teurer.9737/
https://www.hausbau-forum.de/threads/warum-ein-Hausbau-fast-immer-teurer-kommt-als-kalkuliert.16237/
I said at the beginning that we live like 35+.
I'm 45 now. I don't live like that!