ADU's are accessory dwelling units. Not such a sexy word, but the concept is exciting. The data from 2013 shows that the average cost of a detached ADU costs $90,000 and $45,000 for attached (basement) units. Rental income looks to offset the payback period at under ten years with a good income flow to follow. The only only city with more ADU's than Portland is Vancouver, BC.
Aside from potential housing to keep aging parents nearby, another concept of the ADU is to live in the primary house while renting the ADU and when one wishes to downsize, move into the ADU and begin renting the primary house.
Because Multnomah county is promoting denser housing, the normal $8,000-$13,000 fee of System Development Charges have been waived through July 2016.
With the trend of ADU's picking up speed, there is a local blog devoted to the many details of the process and what looks to be a really interesting ADU tour in NE and SE Portland coming up at the end of May which includes a workshop.