Some people still use classified ads.

Look in the classifieds, of your local paper, online, etc. and see what different properties are selling for.

However, if the owner of the house provided a meal to the residents then it is classified as a boarding house. 

If this is what you do and you are a developer/builder then you could be classified as a dealer with inventory so profits would be recognized as ordinary income. 

Look in classifieds then legal notices.The heading will be Notice of Heirship, Notice of Will, Notice of Probate, etc..In CA newspapers the legal notice heading is Notice of Petition to Administer Estate. 

I post on Craigslist, Nextdoor, my work classifieds section, my college alumni classifieds, and sometimes facebook marketplace (but I get more spam and weirdos through that).

@Renee Burke - the classification is very subjective and there is no government organization, board, or company that classifies locations.

See if they can classify it as uninhabitable.

Now I am seeing them stay a lot longer, and many have dropped their prices.Something else I noticed recently - there are LOTS of SFH homes that have guest houses, and the only way to find them is to read the descriptions because they aren't classified on Realtor.com or Redfin as multifamily. 

Some cities might re-classify the property tax from residential to commercial, like when a law office takes over a property that was once a home.