Construction of a granny flat, financing + tax benefits, how to proceed?

However, in our case, the division between main residential unit/secondary apartment is about 80/20, and the secondary apartment is rented to my employer as office space. Because of the smaller proportion of the total volume, the difference in your example calculation should actually shrink, right?

Of course the difference shrinks. Whether all the effort is worth it, you have to decide based on your own numbers. The standard option with the allocation of 20% of the house’s construction costs to rental for depreciation and 20% of the interest as deductible interest expenses remains available to you in any case.

You can also do the loan allocation without dividing the craftsman invoice. Then you just take the risk that the optimized variant is not accepted, and then you still have the standard option with 20% of the interest costs. So I would definitely try that if I were you.

Once again: This is just an amateur opinion.

@ Musketier: Thank you, I will include your summary with our documents, for the financing and later with the tax advisor. @ tomtom79: This is more about the tax aspect. What one should/must consider with multiple residential units or KfW would certainly be beyond the scope here.