Interesting semi-tangent: so I recently moved from the states to Munich, and I live in a little apartment complex in a low-ish density neighborhood (Solln) that has 10 units. Somehow, this place has underground parking, which was initially shocking to me, because I don't think I've ever seen such a small apartment complex in the states with that; usually you'd only seen underground parking in far larger complexes in the middle of a downtown-type area.
And my complex isn't unique, tons of small buildings here seem to have underground parking. So maybe it's not that expensive when it's a common thing to do.
Might also just be a side effect of Munich being one of the highest quality-of-living cities in the world, with some pretty high-end auto manufacturing as well.
No need to make that parking mandatory, but anyone building that stuff will want to include some parking. How much can be left to them. Maybe it makes sense to do underground parking, maybe that is too expensive.