Alright, then let us dedicate ourselves to this thread. I recapitulate: (1) There is a parental property. (2) The property is built upon with a house that is to be vacated by the parents in exchange for the creation of an annex. (3) This "Austragshäusl" annex is assumed here to be legally permissible to be added to this property. (4) The neighboring property is built with the grandparents' house, which is already inhabited by one sibling. (5) You also have another sibling. (7) What kind of compensation took place between you, the sister next door, and the third sibling when the sister moved into the grandparents' house was not mentioned and/or is not subject to the current consideration. (8) Likewise, we do not know whether the sister has already been transferred ownership of the said house or only occupies it. (9) Until now, the spouses of you and the sister next door, along with their respective marital property regimes, are also a black box for the participants in the discussion. (10) We also do not know the age relationships between the mentioned persons / generations, and (11) the intended inheritance arrangement among the parents is not known here yet. From this position, it is still too vague for me to see, for example, whether the proposal of a real estate (development) company between the current and future property users, the third sibling, and possibly other parties involved in later inheritance situations makes sense. Basically, I see company shares as more practical to transfer in tranches matching gift tax exemptions than real estate itself. From all perspectives, it seems sensible to me not to make the newly created Austragshäusl dwelling unit an independent entity in the land register, but rather to specify residential rights on it.