[wongarsu]

Planting grass on top of parking garages isn't that unusual or new in Germany (though usually done for aesthetic reasons), so I assume this is a solved problem.

Sufficiently deep buildings and parking garages might also encounter ground water, so adding soil on top might not even add new problems to the engineering side.

Similarly, the rooftop plants are shown on rooftop designs that would already have to deal with stagnating water

[mkesper]

Municipalities can (and commonly do) grant a reduction of fees for reducing sealed soil, so it's not only aesthetic reasons. https://de.wikipedia.org/wiki/Niederschlagswassergeb%C3%BChr