Single-family house with a granny flat on a 450 sqm hillside plot

If you move into the basement apartment (having your own space is absolutely understandable to me), because grandma doesn't necessarily have to come along (it doesn't look senior-friendly anyway, everything is very cramped) and the non-permanent child moves to the ground floor, you can still downsize the floor plan a bit, have you tried that? The full bathroom on the ground floor could then be completely eliminated, the bathtub would go into the otherwise quite empty children's bathroom.

In my rough calculation, the estimated 350 euros cold rent at an assumed interest rate of 2% and an equally high repayment generated approximately 100,000 euros additional volume. This roughly corresponds to the expansion costs of the basement floor. Therefore, the monthly burden would remain roughly the same under the assumption that one plans with a basement. The calculation looks different in my opinion if one foregoes the basement and builds on a slab. However, with my floor plan, I would then need expensive basement replacement rooms, and the suggestion from Hausbautraum20 with the technical room on the ground floor could be the solution. It is questionable whether our plot allows for building on a slab. I fear that the cost advantage of the slab could be eaten up by the additional costs of the garden landscaping.

In my rough estimate, the estimated 350 euros cold rent at an assumed interest rate of 2% and an equally high repayment generated approximately 100,000 euros of additional volume. This roughly corresponds to the expansion costs of the basement floor. In this respect, the monthly burden would remain approximately the same under the assumption that you plan with a basement. The calculation looks different in my opinion if you waive the cellar and build on a slab. Then I would need expensive cellar replacement rooms with my floor plan, and the suggestion by Hausbautraum20 with the utility room on the ground floor could be the solution.

What I want to say with this:
Without the income from the granny flat, the budget would shrink by about 100,000 euros

Floor area ratio? The development plan does not provide anything there Floor space index 0.4

We basically created the draft on our own. Here and there, optimizations have then been made by the architect.