Sell the property completely or partial sale with investment?

which regulations must be complied with for a multi-family house compared to a single-family house?

I believe that from 3 residential units on, it is a higher building class and up to 3 apartments one would still be allowed to self-manage; if the apartment entrances are above the fourth floor, an elevator is then required or something similar. This is distributed over various regulations, which to my knowledge are mostly uniform nationwide (now also uniformly W/O), at least however the same statewide.

And do you know where one might possibly look up the regulations?

which regulations must be observed for a multi-family house

- higher noise protection (ceilings, walls) - higher fire protection (stair width) But that’s not my area... muenchen.de has a good page... then the reference to the Bavarian Building Code ... well it doesn’t have SN. Has DD, C or L useful internet presences. There should also be a Saxon BO.. have fun reading. Google is the friend... § 27 Load-bearing walls, supports (1) Load-bearing and bracing walls and supports must be sufficiently stable in the event of a fire. They must be 1. fire-resistant in buildings of building class 5, 2. highly fire-retardant in buildings of building class 4, and 3. fire-retardant in buildings of building classes 2 and 3. Sentence 2 applies 1. to floors in the attic only if rooms for occupancy are still possible above; § 29 paragraph 4 remains unaffected; 2. not to balconies, except open corridors that serve as necessary hallways. (2) In the basement, load-bearing and bracing walls and supports must be 1. fire-resistant in buildings of building classes 3 to 5, and 2. fire-retardant in buildings of building classes 1 and 2.

Do apartment buildings have to have a basement? No, right?

Do multi-family houses have to have a basement? No, right?

No, but if you are planning something bigger and also thinking about renting or parking spaces (proof of parking spaces), you build at least with an underground garage plus a few storage rooms/laundry room or something similar.

Yep, storage rooms etc. are needed anyway for the technology. We would like to have checked again whether a conversion of the existing building would be sensible.