Young couple (student and engineer) want to build

That is a great salary you have there, no question, but to finance a house of that size on your own, that won’t work.

How much longer until she earns money?

In about 9 months she will start her traineeship if everything goes well, earning approximately 1200 net. The problem is the location and you won't find this house again, we searched for a long time and want to stay here because our families are here and support us. What you get otherwise is awful, old building from the 70s for 400,000 unsanitary, small plot.

The financial dry spell is thus foreseeable, maximum 1-2 years.

Honestly, I would probably either take in less or wait until she also brings in money. Do you already have a plot of land? Otherwise, it will still take a while before it can get started.

Yes, the property and everything already found, truly dreamy as written above. Exactly that is the dilemma and our small apartment with 40 sqm...

That means you are buying the plot of land and house from the developer? Or separately? If the latter, then buy the plot of land (price?) and tough it out for the transition period until the traineeship in the 40sqm apartment. If the former, then consider that you have to pay property transfer tax on the total price.