Without the separate apartment and with a lot of own work, that might be possible - but definitely not like this (especially in the "expensive" south).
Is this the offer of a construction company or are the figures roughly estimated? The ancillary construction costs are definitely set too low. Better increase to €50,000, if you don't need them all the better.
So we currently have two offers for turnkey (wood frame) in South Baden. Approximately the same conditions and our price is €400,000 including the basement and additional construction costs. Land with around €100,000 not included.