This is pretty common in areas of Seattle. The in-law unit is legal with permits and in the basement of the home with a separate entrance. The homes sell at a discount compared to a normal configuration. Are these a good way to go if buying with the intent of turning it from a primary to rental in 2-3 years? The rental yield is way better than a standard SFH. It’d be pretty easy to convert back once everything is worn out and needs replaced anyway. Seems like a way to collect higher rents and still get top dollar selling (assuming after 20-30 years it’d take some remodeling money to sell either way).
Most of them show up you search for “ADU” in remarks:
https://www.redfin.com/city/16163/WA/Seattle/filter/property-type=house,remarks=ADU