Residential construction on existing building - parents' property

For the project, you need an architect who will create a preliminary plan and cost estimate for you. There is also a good website from [MMST Architekten Hamburg] with an e-book on the topic of [Zinshaus]. It is also important to know the market situation. Are rental apartments in demand in the area? How high is the rent per m2 (it helps to analyze the real estate portals). If you have the construction costs and the rental yield, the yield calculator from [Stiftung Warentest] is helpful. It also takes tax effects (depreciation) and vacancy into account. This will give you a possible yield for the multifamily house. And then you still have to consider whether you want the work of "renting out." With 4 apartments, there will regularly be inquiries/requests from a tenant or new rentals.

And, if something is to be built "on top" somewhere, the static stability of the "underlying structure" must first be checked, because it is not necessarily suitable for this construction. So ... call in an expert.