I marked it with 0 levels because I used building parts with the actual building levels to make a 3D version of that buildings. You can view the 3D buildings at: https://demo.f4map.com/#lat=48.0491888&lon=8.2083110&zoom=19&camera.theta=63.671&camera.phi=35.237

More Information about 3D buildings:
https://wiki.openstreetmap.org/wiki/Simple_3D_Buildings
https://wiki.openstreetmap.org/wiki/Key:building:part
https://wiki.openstreetmap.org/wiki/3D