Thanks for the note. You are totally correct; I added the building contures, and assigned operator=Agon Verwaltungs GmbH to the area around the garages.
Besides this I am not sure how to correctly add the further details and what could be the main tag:
at least the following tags could be set 

contact:phone=+49 4471 8504226 contact:website=http://www.agon-clp.de/box/
description=Mietgaragen und Lagerräume
name=ACON
operator=Agon Verwaltungs GmbH