According to wiki you should respect the original use of a building. I guess that goes for there garages as well? "The most basic use is building=yes, but the value may be used to classify the (architectural) type of building. Note that it may be not the same as the building's current use (tagged using building:use=*). For example, a hospital building that is abandoned or repurposed to be a marketplace is still a building=hospital, and to mark active hospitals amenity=hospital is used." ... as they explain here ... https://wiki.openstreetmap.org/wiki/Key:building