Hello,
Divide the total costs completely proportionally between the owner-occupied apartment and the rental apartment
For this, the tax advisor will tell his client, the OP, exactly "how, what & where." For the later general contractor / main contractor, it means more work upfront (adjusting the contract for two residential units), but from then on, everything runs smoothly. With individual awarding of the trades, however, there will continuously be additional effort in invoicing, allocation, and control. The contracted craftsmen will surely be happy :cool:
Rhenish greetings