Yes. I know StreetComplete has limited options. I use it myself. What exactly do you mean by "rooms". Some kind of shed or maybe residential use of building guest houses etc. Personally I would still use building=garage as it was probably the iniitial "and main" useage of the building. Anyhow I would only use the address with the main building. This same subject has been touched upon in this discussion ... https://wiki.openstreetmap.org/wiki/Talk:Tag:building%3Dgarage