Portland has seen a six-fold rise in the number of accessory dwelling units (ADUs) in the past few years, and more of these small, separate living units are on the way, according to the Oregon Department of Environmental Quality and several of its green building partners, which include Metro and the City of Portland Bureau of Planning and Sustainability.

ADUs are additional, separate living units on single-family lots. They average about 665 square feet in size, according to a recent DEQ survey of accessory dwelling units in Portland, Eugene and Ashland. They can be converted basements, attics, garages or entirely new backyard cottages. Sometimes called “granny flats” or “mother-in-law suites”, they can provide extra income for property owners, as well as multi-generational housing for people who simply don’t need a lot of space. They’re allowed in all 25 cities in the Portland metro area and in many other Oregon cities, with Salem the most notable exception.

In Portland, the number of ADUs permitted in 2013 was six times greater than the yearly average from 2000 to 2009. This dramatic increase is the result of a 2010 City of Portland fee waiver, which reduced the cost of building an accessory dwelling unit by $8,000 to $11,000. Before the waiver, about 30 ADUs were built in Portland annually. Recent data from the City of Portland lists almost 200 ADU permit applications in 2013. The waiver’s popularity caused the Portland City Council to extend the waiver until July 2016.

DEQ’s interest in ADUs stems from a 2010 project that compared the environmental benefits of 25 different green building practices. The research results, which were used as policy support for the 2010 Portland fee waiver, indicate that reducing the size of a house was the most effective way to reduce both the energy and material-related greenhouse gas impacts of a house.

To further encourage accessory dwelling unit housing, DEQ and its partners are sponsoring a tour of 12 Portland ADUs for people who want to learn more about permitting, financing, designing and building space-saving, low-impact units.

Sunday, June 1 a tour, organized by Caravan: The Tiny House Hotel, will provide a rare opportunity to see ADUs and meet homeowners, builders and designers who built them.

DEQ and its partners have produced a three-minute animated video to increase awareness of ADUs. Additional information, the video and a compilation of ADU development case studies, is available at accessorydwellings.org.

Contact Jordan Palmeri, DEQ green building program coordinator, at 503.229.6766. Information about the upcoming tour will be on DEQ’s website www.oregon.gov/DEQ