Hey Sean,
I was trying a similar experience house hacking in tacoma and come to find out that most applicable regulations are from the city and not the county/ state. If you search for Bellevue WA boarding house, you find the city regulations on boarding house as "Boarding houses and bed and breakfasts require a Home Occupation Permit, Part 20.30N LUC, approval. In addition, not more than two rooms may be rented to not more than two persons other than those occupying a single-family dwelling, provided there is compliance with health and building code requirements. The owner of the rooms to be rented shall provide off-street parking for such rooms at the rate of at least one parking stall for each room"
Found at https://bellevue.municipal.cod...
I also read about a restriction on zones for boarding homes. You can find the zone map here
https://cobgis.maps.arcgis.com...
There is a chance that you could do a non-occupied rooming house which matches closer to what you are looking to do. Here's some more info
https://bellevuewa.gov/city-go...
Good luck and let us know what happens!