What qualifies as an ADU

[Sajeev A.]

I'm getting mixed messages on what's an ADU and wanted to clarify.

I've a room that is currently used as a media room and is connected to the home by a double door. I'm thinking of closing off that door and converting a window into an exterior door. After adding a small washer/dryer, sink, dishwasher, cabinets and a countertop, the plan is to give it out as a rental. Would that make it an ADU? In that case, seems like there are a lot of approvals required from the county and taxes to be paid. Before I embark on this expense and process, just wanted to get some insights on whether this is actually an ADU. Would I need to consult an attorney? I'm based in Portland OR.