In my city in Virginia, this is a requirement. I do not know if Washington State has the same. We pay monthly taxes and lodging fees to the city and receive tax documents at the end of the year. Since my yearly AirBnb income is minor (comparatively speaking to most larger hotel type establishments) my liscense cost $50 a year. With regards to having multiple listings, I would be curious if you would be able to deduct expenses for the properties and depreciate at tax time. I only have one room I let, so really haven't worried about that side of the equation but for someone in your situation, this could be a benefit.