Is more than 0€ tax-free?
If you received the living space as a gift or inheritance, certainly.
But generating living space at 3000 euros per m2 to get 10 euros cold rent yourself is initially 4% gross. Minus interest. Minus the lifestyle feeling of a condominium in a multi-family house. And possibly extremely annoying tenants who walk all over you – yes, even if they live on the lower floor.
But the point is much more: If you yourself can just finance your own apartment, why should you get involved in something like this at all? It is a pronounced concentration risk on top of that! To make it work, your own apartment has to be smaller than it otherwise could be.
Then I would rather turn the tables and see if two single-family houses or a semi-detached house are possible on the plot.
With a standard land value of 250/m2, market prices should rather be around 400, correct? You could talk to a general contractor (GU) to see if they could build a semi-detached house or a single-family house for significantly less equity and in return get the rest of the plot for resale purposes. As a simple calculation: 400 separate m2 could possibly yield 150k euros, and with construction costs of 350k euros, the resulting 200k euro loan with an 850 euro repayment fixed for 20 years would be feasible and then paid off completely(!). That would leave "only" 500 m2, but that is quite normal in many building areas nowadays.
So if I didn't necessarily need the 900 m2, I would know what I’d do... With an 850 euro repayment, the original poster would only pay about 300 more cold rent than currently, so that would be easy-peasy. Then you don't just live for the house but also live like that – and can later provide something for the children. And the house itself could be made larger too.