In 2010, during the economic slump, when many building plans were being shelved, Portland presciently began to allow homeowners the right to develop accessory dwelling units on standard 5,000-square-foot residential lots for the first time. The city also eliminated development charges of up to $15,000 for new accessory dwelling units to spur homeowners to build.  More incentives followed: Homeowners could build and even rent out a unit that did not have off-street parking; any design not visible from the street could be built without input from neighbors; and new height limits — raised to 20 feet from 18 feet — encouraged two-story units, like Ms. Wilson’s.  “All that helped,” said Shawn Wood, a construction waste specialist at the Portland Bureau of Planning and Sustainability and an expert on accessory dwelling units. “We went from about 35 A.D.U. permits in 2010 to 250 in 2015.”  “The benefits of cities growing inward are many,” Mr. Wood said. “People can take advantage of existing infrastructure and maintain old neighborhoods.” And residents can downsize and age in place.  The Garden House has been influential, especially on the West Coast.  According to Daniel Garber, an architect in Palo Alto, Calif., and former city planning commissioner, “most Bay Area communities now allow A.D.U.s. Only Palo Alto has not yet caught up with its northern neighbors in liberalizing A.D.U. construction.”  Not surprisingly, the concentration of accessory dwelling units has been in central, higher-income areas close to amenities like transit and shops. “Part of this could be due to the fact that people with large amounts of equity can more easily secure financing,” Mr. Wood said. “The City of Portland and Portland State University will be working on a project to encourage and facilitate A.D.U. development in more diverse neighborhoods.”  There are certainly architects and planners who do not see these accessory dwelling units as real solutions to any housing crunch. Richard Potestio, an architect in Portland who is interested in multi-unit housing, said he believed that larger, more efficient and less expensive accessory dwelling units could be incorporated into new multi-unit buildings.  If a condominium, like a single-family home, could have an accessory unit, he said, a 10-unit condominium building could have 20 dwellings. And while multi-unit buildings are allowed in commercial zones, Mr. Potestio said that similar rules for residential lots would help the environment. “When A.D.U.s are incorporated with duplexes that share common walls, less open permeable space is lost,” he said.  Accessory dwelling units are changing lives in other unexpected ways.  Bryan Scott, 40, a software designer and technology consultant in Portland, and his wife, Jen Wantland, 42, an apparel industry planner, quit their jobs in 2012 to travel in Mexico but returned earlier than planned, and found that the renters in their home could not shorten their stay. Lacking affordable alternatives, Mr. Scott and Ms. Wantland decided in late 2013 to convert their 480-square-foot garage into a pied-à-terre. “Until then, we had never heard of A.D.U.s,” Mr. Scott said, speaking recently from his 1967 Volkswagen van while traveling in British Columbia.  In fact, with the newfound ability to take off when they please, they have become modern nomads, and the van is often their home/office on the road. Another dividend: At Zenbox Design, a firm they recently founded, they are both planning and design consultants for others interested in building accessory dwelling units.  In Emeryville, Calif., Adriana Daringa, 41, and her husband Benjamin Corotis, 41, are architects who run the firm ADBC Architecture. In 2003, they purchased a 6,063-square-foot property with two small houses, which they painstakingly remodeled. The houses were legal because they predated zoning rules, but under new efforts to allow accessory dwelling units, they would now be allowed in newer construction.  Ms. Daringa and Mr. Corotis live with their 3-year-old daughter, Alexa, in the older of the buildings, a 100-year-old, 1,200-square-foot house that faces the street. Mr. Corotis’s sister, Lindsay Cortis, 39, who is a building contractor, lives in the 900-square-foot 1950s cottage in back. They share tandem parking spaces and the central courtyard.  “We wanted to extend our living space to the outside,” Ms. Daringa said. So, in lieu of a mudroom, they built a breakfast room with doors to the garden that let light into the refurbished kitchen, too.  “It was a lot of work. We were only 28 when we started, and naïve about how much work it would be,” Ms. Daringa said. But sweat equity kept costs under $215,000.  In some ways, Ms. Daringa said, the result reminded her of her grandmother’s place in Romania, where she grew up.  “We have created a small family compound,” she said.  An earlier version of this article misstated Daniel Garber’s former occupation. He was a city planning commissioner in Palo Alto, Calif., not a city planner.
The Oregon city has developed solutions to the lack of affordable housing for the middle class, and the ideas are being embraced in other cities.