DC and Portland both tried to encourage accessory apartment construction. Why does Portland have so many more?

In a 2016 update to its zoning code, DC allowed by-right development of Accessory Dwelling Units, or ADUs, in most residential zones. Today, many DC homeowners are allowed to build and rent out in-law suites, basement apartments and other secondary housing on their property without getting special permission.

Another city, Portland, Oregon, also loosed restrictions on ADUs (not to be confused with Affordable Dwelling Units) in the last decade, passing a waiver of infrastructure fees for building accessory units in 2010.

Why did accessory apartments catch on in Portland but not DC? A new study from the Urban Institute compares the land-use reform process in each city, as well as the end result.

Both cities have some comparable history when it comes to housing. Portland and DC are similar in population size. Like DC, Portland saw a 24% rise in population between 2000 and 2019 and needed more affordable housing options. Like DC, Portland saw ADUs as a possible solution.

But that’s where the similarities end: while Portland’s accessory units skyrocketed, with hundreds built each year, DC’s accessory apartments saw a much smaller bump. From 2017 through 2019, Portland issued more than 1,500 ADU building permits; DC issued only 68.

Though DC faced structural and historical issues making ADUs more difficult to implement, researchers Lydia Lo and Eleanor Noble also found that each city approached the reform process differently. The result is that building an accessory apartment is far more costly in DC than in Portland, and few people find it’s worth their while.

Incremental or all at once?

In fairness to DC, Portland had a massive head start on accessory apartments — it launched a (relatively restrictive) program in 1981, adding them to the zoning code 10 years later.

Since then, Portland has incrementally loosened restrictions on ADU construction. The city removed requirements that the primary residence be owner-occupied between 1997 and 2003. In 2004, the city allowed garage conversions and reduced setbacks. And in 2010, it passed a provision adjusting minimum size requirements to let homeowners on smaller lots build accessory units.

Because accessory apartments had existed in Portland for years, advocates told Urban Institute researchers that opposition was scarce — the units that had already been built had caused no major disruptions.

DC’s process differed substantially. Before 2016, ADUs required special exceptions and hence were rare, with only about one built each year.

Accessory apartments were proposed as part of DC’s zoning regulation review, a process that began in 2007 to simplify the zoning code. That process was supposed to take three years, the Urban Institute report says. Instead, it took nine, expanding into years of community engagement on an enormous package of issues including parking, retail and building height. DC planners and other officials told Urban Institute researchers that the large package of reforms made the process seem like a “much larger and riskier project.”

The process leaned heavily toward community input, but Urban Institute researchers found that community members who submitted testimony did not represent the District as a whole. Participants were often wealthy and leaned toward maintaining lower density. Only one person from Ward 7 submitted testimony on accessory apartments, and nobody from Ward 8 did so.

Portland’s community input process was also made up of disproportionately white and wealthy residents. But upon finding their sample was not representative, Portland officials “instead based their decisionmaking on more representative, supplementary survey data,” the researchers said. This data included surveys of ADU owners by Portland State University, finding, for instance, that owner-occupancy requirements were ineffective, and that most ADUs were long-term rentals, the study says.

In DC, on the other hand, officials “gave roughly equal weight to supportive and opposing comments and added expensive restrictions to the reform that only 27 percent of commenters (almost entirely from wealthier wards) requested.”

A costly process

In the end, DC’s new zoning regulations passed allowing accessory units by right in residential zones. But those regulations are more restrictive than advocates wanted.

The Urban Institute researchers write that ADU regulations are too complex for many to navigate without hiring lawyers and architects; the space requirements cut out many potential builders; and the rules require the owner to live on the property.

Though external accessory apartments are allowed anywhere on a property in zones coded Residential, Residential Flat zones have more restrictions, said Cheryl Cort, Policy Manager for the Coalition for Smarter Growth. Homeowners in those zones, typically rowhouse neighborhoods, cannot build units that are not attached to the primary residence without a special exception, a long and often costly process.

Cort said another challenge in DC is that most external accessory units have to stay within a small footprint of only 450 square feet — too small, she said, for most people to build apartments in which people can age in place using mobility devices like wheelchairs.

A forthcoming Coalition for Smarter Growth publication estimates the cost of building a 450 square foot accessory dwelling in DC. Together, “soft costs” for architectural and engineering work and construction and infrastructure fees are estimated to cost between about $24,000 and $36,500 — around 20% of total costs.

“With so many costs in the ADU building process, it is unsurprising that production has remained low,” the Urban Institute report says.

Cort said one major barrier to accessory apartment construction is helping individual homeowners, who are not professional developers, navigate the process of designing, financing, permitting and building housing units.

Over the last few months, she said, officials have been working to streamline permitting, trimming the process to less than a month in some cases.

Other ADU education initiatives for DC residents include webinars and an email forum.

Despite the challenges around education and cost, Cort sees accessory apartments as a promising way to help people, particularly low-income longtime homeowners, build wealth. But making the process easier and bringing costs down will take multiple strategies. ”We don’t really have a silver bullet,” Cort said.