Multigenerational house with 3 families

Hello,
You have 2 children, both of whom have families with children.
So a smaller apartment is to be built for you and one apartment/semi-detached house each for the siblings with their families?!
May I ask how large these families are?
100 sqm for 3 people can already be tight if a cellar replacement room is integrated into each “apartment”. If more children come, it will become too cramped again.
Basically, a 3-family house can be designed in several ways: e.g. 80 and 100 sqm on the ground floor, 140 on the upper floor. Shared freezer room.

Founding a [GbR] is feasible.

The consideration arose from the fact that our house has now become too big for us and we would like to downsize. The children have families with grandchildren. They have apartments that are too small.

How about the following option: one of the child families "takes over" the parental home and you move with the other child family in the town or neighboring town into a house with only one granny flat. This is also intended as a reduction of complexity, after all, three-generation projects usually do not occur in the minds of housing development planners.

Formation of a GbR is possible.

In a constellation with inheritance entanglements between the partners, however, to put it mildly, not trivial – even with the same marital property regimes in the two branches of F1 :-)