I wonder what two floors are good for?! When I get out of the car, I use the lower one; when I am without a car, even if only in the garden, the things I need upstairs are missing because they are downstairs. Aside from that, I would actually do without such a staircase in a bungalow. You make it the main feature of the house here. "Bungalow built around stairs – the emergency stretcher gets stuck" could be a headline. Furthermore, it remains unclear where the ground-level storage is for the office, sewing machine, etc.
I would actually build a bungalow here, but without a basement if money is to be used sparingly. In the north, it will sink somewhat into the slope, but that doesn’t matter if you place the utility rooms there. If necessary, you can use skylights; they provide nice light. In addition, a double carport on the side where the main entrance is and where the mailman rings. Without barriers!
If money is available, a bungalow with two levels...
By the way, I will also correct the 2000/sqm here, as the 2000 refers to an average, and a bungalow is relatively more expensive than an average house. A garage inside the house also makes a basement more expensive, see insulation. Then, of course, a terrace/balcony on a basement connected to living space also costs more than a normal terrace and a normal basement. So: I can imagine an energy disaster and thus much higher costs with this basement/living space/garage/terrace combination.
Gemeinsame monatl. Ausgaben (Gesamt): ca. 2900.-
Gemeinsames monatl. Nettoeinkommen: 4000.-
Without rent! Where does your money go? If I were you, I would think about that. We don’t even have 2900 including financing, etc….
I would not include the rental income at all but consider the 3-family house separately.