Apartment above two plots

Hello, We plan to build a duplex with my brother. Each of us will get his half. In the basement, there should be a granny flat for our parents. Since the granny flat should be about 70-80 sqm, one half is not enough. Therefore, we wanted to "build" the granny flat on both halves. But how will the granny flat then be divided between us brothers?!

Phew... This is unusual, but not impossible.

Option 1: A shared large house with 3 residential units, basically like condominiums. Option 2: Somehow through usage rights, but I only know this from a townhouse project where a homeowners' association has the right to operate a shared underground garage beneath the gardens, whose plots are individually owned. Option 3: Internally within the family, everything shared from one pot. Carries corresponding risks but can work well. A colleague has been doing this for years.

I would tend towards Option 1 within the family if you want to do it properly right from the start.

Is it even permitted by the bb to build 3 residential units?

per semi-detached house 2 residential units
per single-family house (in case of merging) 3 residential units

That’s why we wanted to “develop” the apartment in both halves. But how will the apartment then be divided between us brothers?!

In my opinion, this is an absolutely terrible idea, and I already said elsewhere that no one will agree to distribute a residential unit over two plots. I also recommended going to the authorities to have the plots combined and then to have the total property approved for the previous number of residential units (two for each, so four in total). What can be done is to install a connecting door between both houses, so that the parents would then have two apartments (both parts would have to be pre-equipped for bathroom and kitchen). For example, you build a bathroom into one part, and a kitchen in the other part, leaving both “kitchens” still unrealized.

Yes, the office here is not that fast... we have only just now been allocated the plots of land. No one really wants to make any decisions there. But thank you very much, that already helps.