We actually do have a distinction for that in MLS; contact me if I can help--
While most "3rd party sites and websites" don't allow this search-feature, we have it within our own MLS, (in additional to a lot of other search-criteria) so that you don't have to waste your time sorting through "junk" that doesn't fit your needs. It's a search-term called "guest-quarters" and is located under "exterior features".
-Amy S. Arey, Realtor
Halo Group Realty, LLC