How many square meters am I allowed to build and what are the costs (near Munich)?

Wouldn't a multi-family house also have to be more expensive because you have more bathrooms, apartment doors, kitchen connections, and that kind of stuff? Elevator? Well, that might be somewhat offset by a higher share of hallways and staircases. But not necessarily completely.

From three residential units, playgrounds must even be built. BayBo: Art 7 Abs. 3

Wouldn't a multi-family house also have to be more expensive because you have more bathrooms, apartment doors, kitchen connections, and such stuff? Elevator?

Upscale in a multi-family house is not as upscale as in a single-family house. There are also many synergy effects. Current example from a metropolis in multi-storey residential construction. Pure construction costs - nothing else - at €2,950 per sqm/GFA, so about €3,400 per sqm/living area. We are really talking about upscale - sales price at €16,000 per sqm + underground parking space €50,000, which is included in the construction costs.

From three residential units onwards, one must even build children's playgrounds.
BayBo: Art 7 para. 3.

There is a relatively recent case in Erding where the municipality then demanded a €60,000 payment in lieu. No idea how it ended.

Wouldn't a multi-family house also have to be more expensive because you have more bathrooms, apartment doors, kitchen connections and stuff like that?

How do you come up with that nonsense? - only one kitchen per family, backup kitchen and children's bathroom are not yet included in just "upscale".

No, I meant per sqm not per family. Because the apartments are smaller, but each apartment has at least one bathroom and a kitchen.