Hi Jacky,
This is a pretty common scenario you're in. Especially in our BP world of "house-hacking." Quick definitions:
- Mother-in-law: simply means the basement of a home has a kitchen, separate entrance, and a dividing door or something like that
- Accessory Dwelling Unit: means the city has approved the ability of the owner-occupant to rent out a portion of their home. "ADU" legality is typically defined by each city individually. If they are allowed, they typically require a business license or something like that
- Duplex or Multiunit: means the property is zoned and legally permitted to have multiple families living in the same structor and does not require an owner-occupant
It sounds like your home with the MIL will need city approval to be a legal ADU or a zoning change to become a legal duplex. It's rare that a city will change the zoning for a single family to be multi-family but I don't know the specifics of the Washington market. I've heard they're become really lenient on ADU and multifamily so you might be in luck!
My recommendation would be to go to your county website and look at the zoning and try to make a game plan from there. It's unlikely you'll have a quick solution for the duplex-like income unfortunately but you could always convert the rental into a single family unit and rent out the whole thing as a quick solution.
Good luck!