Accessory Dwelling Units and Beyond

Secondary units on single-family-zoned lots, formally known as Accessory Dwelling Units (ADUs) and more commonly as granny flats, garage apartments, and backyard cottages, help increase housing supply, make it affordable, and layer in density within the prevailing fabric of single-family neighborhoods. However, most cities are reluctant to permit ADUs easily because of real and perceived homeowner opposition. Scholars from both market-oriented and social housing-based perspectives have become skeptical and more critical of neighborhood-based opposition in cities to new housing developments. Given the United States’ federalism context and the lack of strong metropolitan institutions, scholars and policymakers concerned about housing supply in cities see state intervention as the most promising arena for addressing local barriers. While California has used state preemption to allow ADUs in cities throughout the state, the local governments of Seattle, Portland, and Minneapolis have implemented incremental changes to allow secondary units on single-family-zoned lots. Minneapolis went a radical step further and became the first major U.S. city to allow three units on all single-family and duplexzoned lots. Portland also upzoned single-family neighborhoods to allow duplexes, triplexes, and multiple ADUs, and Seattle allows two ADUs on single-family-zoned lots. Through inductive case study research, we research the institutional processes of Seattle, Portland, and Minneapolis to understand under what conditions local governments can overcome opposition to ADUs and how ADUs can become a gateway to higher-density cities. The research project adds to scholars and policymakers understanding of the appropriate scale for decision-making regarding housing and zoning regulations and how local governments can work with parochial communities to proactively overcome housing supply barriers.