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Research report2017Portland, Seattle, and Vancouver

Jumpstarting the Market for Accessory Dwelling Units

Chapple, Wegmann, Mashhood & Coleman

Main finding

ADU production depended on a bundle of usable zoning, flexible occupancy and parking rules, manageable fees, financing, and technical help.

Portland issued nearly 2,000 ADU permits after 2010

Most important limitation

The study is descriptive: policy, demand, and homeowner selection changed together, so the permit count is not a causal estimate.

From the publication

Inspect the authors' figure directly

Study chart · redrawn

Figure 2 · ADU permitting and reforms in Portland

The paper places parking, occupancy, design, fee, and education changes directly on the permit timeline.

REDRAWN FROM STUDY DATAFigure 2 · ADU permitting and reforms in Portland
Figure 2 · ADU permitting and reforms in PortlandThe paper places parking, occupancy, design, fee, and education changes directly on the permit timeline. Values digitized from Figure 2; alternate-year values shown to keep the mobile chart legible in the archived publication; minor reading error is possible.0200400600ADU permitsYearAnnual ADU permits200020022004200620082010201220142016

Source: Chapple et al. (2017), Figure 2; PDF p. 9.

Read carefully: The timeline is descriptive: several policies, financing conditions, outreach efforts, and housing demand changed together.

How this study is used

Claims this evidence supports

Strong evidenceLegal on paper is not the same as buildable in practice.See the full evidence stack →

Policy connection

Solution guides informed by this study

01Make room for apartments, shops, and neighbors02Bring back the homes between a house and a high-rise03Let homeowners add a home04Stop forcing every home to buy parking06Set clear rules—and say yes on time

Reference

Chapple, Wegmann, Mashhood & Coleman (2017). “Jumpstarting the Market for Accessory Dwelling Units.”

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