I also just recently purchased my 2nd investment property in September 2015: a 3-bedroom student rental house with a 2-bedroom accessory apartment.  

Researched uncovered that Accessory Dwelling Unit (ADUs) are allowed in R2.. yet seems to require a $4000 statement of occupancy.  

Another unique thing about the city of Seattle specifically is that Seattle allows for a single family home to have 2 ADUs (accessory dwelling units), one of which can be detached.

Right now I live in the main part of the house, and rent by room to a group of 3 people, the accessory apartment is also rented.

@Russell Brazil what about creating an ADU (accessory dwelling unit) indirectly converting a single unit to a 2 unit. 

I'd probably look for an area that allows for accessory apartments or try to do some kind of duplex.

We did a very nice total remodel and it is stocked very well- i.e more than 4 forks etc and tenants really appreciate it... coffee maker, spices, quality cookware, bikes, beach accessories.

Great question - our experience has been that a DADU has to be built as an accessory structure to a home.

In zoning districts where no more than three unrelated persons may occupy a dwelling unit (as specified in subsection 175-56 General Dimensional Standards), there shall be no more than three unrelated occupants in total for the single-family dwelling and the accessory dwelling unit combined or for the single-family dwelling and the accessory apartment combined."

Even allowing increased density within existing structures or modest infill development (accessory dwelling units or "granny flats" in backyards).