How to get money in the "most favorable" way

The whole idea is just silly... it fails on several points:

- You have no equity (no, 20k EUR is practically nothing)
- Your income is too low & you are too young (which of course are related)
- you won’t get approval for a “hall” as living space... garage downstairs & living upstairs come with very strict regulations (fire protection, etc.)
- no bank will go along with it anyway
- with the budget you presented, it is never feasible.. easily 100k-200k

Work for a few years and save up real equity, then rather try to realize a residential building + large garage next door (e.g. prefabricated building)... then it will also work with a budget...

- you don't get a "hall" as living space

Or alternatively build it as a commercial hall with an attached apartment. Essentially the same as planned by the OP, just a different label and you could leverage tax synergies. That can also be marketed differently to a bank ;)