I also think that in Saxony a house ready for occupancy for 300,000 EUR (possibly even 250,000 EUR) should be feasible, but of course plus additional costs. And those are a wildcard. If you have difficult soil conditions, they can easily get out of hand. I have a completely flat plot and still 6,500 EUR just for the foundation earthworks! And I didn't have any excavation removed because I hope to be able to distribute it later. Together with the earthworks for the utility connections, another 10,000 EUR could easily be added without personal contribution. And I have almost optimal conditions. So starting a conversation with 500,000 EUR to see if it would be possible is at least not a mistake. You can always aim lower when it gets a bit more concrete. Certainly possible with existing buildings as well.