The Connection Between Unpermitted ADUs in San José and the Need for Zoning Reform

The affordable housing crisis continues to worsen, with rising homelessness and displacement coinciding with economic hardship. In response, many homeowners are building additional housing units to generate rental income or house more people.

However, many of these units are constructed without permits, posing a challenge for both policymakers who to assess the impact of housing policies and homeowners and tenants who are exposed to l safety risks.

In "Not (Officially) in My Backyard: Characterizing Informal Accessory Dwelling Units and Informing Housing Policy With Remote Sensing" (Journal of American Planning Association, Vol. 91, No. 1) Nathanael Jo, Andrea Vallebueno, Derek Ouyang, and Daniel E. Ho leverage computer vision to estimate the number of detached accessory dwelling unit constructions in San Jose, California.

The Rise of Accessory Dwelling Units

Accessory dwelling units (ADUs) are more affordable, quicker to build, and offer flexible living options compared to primary residences. They provide homeowners with rental income or space for family and friends, increasing the housing supply without altering neighborhood character.

However, many ADUs nationwide are built without permits, likely due to complex regulations and high costs. While some states, including California, have passed laws to ease the ADU permitting process due to its critical role in addressing the housing crisis, the lack of data on unpermitted units limits our understanding of how ADUs are being developed.

Figure 2: Example of a permitted detached ADU built during 2016–2020. On the left, is a 3D visualization of the ADU and the parcel in which it is located using Google Earth Pro; on the right, is the annotated National Agriculture Imagery Program (NAIP) imagery from 2016 and 2020 for the parcel, including an orange polygon around the ADU in 2020 (Google Earth, 2022; Earth Resources Observation and Science Center, 2022).

Using computer vision technology and human annotation, the authors estimate how many ADUs were built between 2016 and 2020 in San José, California. By comparing these estimates with city-issued permits, they found that 78 percent of detached ADUs, about 1,045 units, were unpermitted.

These unpermitted ADUs are more common in less affluent, denser, and more diverse neighborhoods. This finding helps explain the gap between San José's housing unit count, which is based on permits, and the higher numbers reported by the U.S. Census, which relies on survey data.

A Call to Address Housing Crisis

The high rate of unpermitted ADUs suggests that many homeowners have taken it upon themselves to address the housing crisis in the absence of effective policy solutions.

To further promote legal ADU development, it is essential to identify the main barriers homeowners face in code compliance and the permitting process, especially in less wealthy and minority communities that encounter greater challenges.

Learn more about the importance of planning-led zoning reform and APA's top policy priority.

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