Floor plan first to live in, then to rent out, but how?

Unproblematic. Especially due to the separate entrance areas, there is a reasonable separation. When building, be sure to pay attention to sound bridges and, if in doubt, use 24 cm KS solid blocks instead of 11 cm bricks for the separation.

That it is planned as a two-family house is obviously unproblematic. How does it prove itself in everyday life with small children when the 2 residential units have been combined? The floor plan is not modern and open, but not everyone wants that either

The entrance area can be quickly separated or reopened using drywall.

Especially due to the separate entrance areas, there is a reasonable separation.

It does not prove itself in family life: staircase at the entrance/dirt area, larger hallway separates the children's rooms and bedrooms too much from the ground floor living area. This stairwell is certainly not cozy, nor is it contemporary anymore. I occasionally visit old country houses where it actually works separated like this. But no one today wants to live their family life with many doors and a closed hallway unit. At least this is what is communicated here and elsewhere when it comes to childcare, home office, open space, etc. And you say it yourself: you first have to spend significantly more money on building – with the disadvantage that you do not live alone on the property and that renovation is needed when the child comes. There are two possibilities to weigh: having tenants or being private!

I actually have to disagree with my predecessors here, since I know 3 properties in my close circle of friends that have recently implemented it. In 2 of the 3 cases, the additional residential unit is currently (“before the children”) even already rented out. In one version, the upper floor consists exclusively of the 3 children's rooms + bathroom, which can easily function as a 2-room apartment with kitchen. The entrance area can be quickly separated or reopened by drywall construction.

Thank you for the counterargument – it would be even better if you illustrated it with numbers, what this means in terms of benefit vs. additional effort in the mentioned cases. That little drywall is “only half the battle”: a controlled residential ventilation system is voluntary in itself – but if you do have one, separation per residential unit is mandatory. Do you want to forego this in the convertible area, or do you want to undertake the significantly increased effort?

In the other version, 2 large rooms plus a bathroom are planned for the children, currently accessible via an external staircase, representing a 40 sqm apartment. Also quickly reversible.

That would then be like in the semi-detached house of the brothers – see: (a miserable substitute for the originally unknown disappeared thread).