Rules and laws with ADU's

[Shawn Tompke]

I'm a beginner about to purchase my first property. I'm looking at a property that has a 3/1 stick built house and a doublewide (2005) ADU on it. It is on about an acre of land. This is in WA state, thurston county. I have already had a few different lenders say they wouldn't loan on a property with an ADU. I think I've found one now and I'll be using an FHA loan. I'm just wondering if I would be able to rent this ADU out legally? Or what questions I would need to ask to make sure that I can. Thanks so much!