Adding a floor to someone else's multi-family house

Hello everyone, I recently had the idea of adding a penthouse to an existing apartment building in times of unaffordable plots of land. Ideally, the owner has damage to the roof that would need to be renovated anyway. I would be interested in your opinion on this idea. Completely absurd or quite conceivable? Of course, I would offer the owner amount X, but I'm not at that point yet... looking forward to any assessment!!

I would be interested in your opinion on this idea.

Legally, the apartment then belongs to the third party and not to you. Provided the apartment building is already financed, you also won’t get an additional loan. The only feasible option: division of the apartment building including the newly to be built penthouse into condominiums. You then own the penthouse (which still has to be built). And is there an owner who would agree to that? In the end, you would be entirely at the mercy of the seller. If he later decides that he wants to insulate the building, he will do it and you have to pay for it. Ultimately doable, but still absurd and legally as well as structurally (are you even allowed to build upwards? Is the structure suitable for it?) a huge challenge. Not to mention the costs – acquisition of owner’s association shares on the property, adjustment of water, drainage, electricity, heating, and all in accordance with the Building Energy Act (does the rest then also have to be renovated energetically?!).

Hello everyone, recently I had the idea to put a penthouse on top of an existing apartment building in times of unaffordable land.

You are not the first, we have had that before:
https://www.hausbau-forum.de/threads/planung-aufstockung-fundament-traglast-alternativen.32439/