Factors Associated with Accessory Dwelling Unit Density

Abstract

Accessory dwelling units (ADUs) are self-contained living quarters with kitchen and bath, found in single family properties. These quarters are either attached to the home or are detached structures on individual lots. Zoning codes often prohibit ADUs in single family neighborhoods. Advocates for ADUs have championed their legitimacy arguing that they offer low cost, parcel based solutions to increasing the stock of affordable housing and the need to accommodate elderly family members. ADUs have been legal in Washington State in most cities since 1995. Some cities have seen greater installation rates than others. This research explored the socio-economic, socio-demographic and built environments factors associated with ADU density in four cities: Kirkland, Mercer Island, Sammamish and Shoreline. Logistic regression modeling was performed analyzing factors on a census block group level. Results suggest that the density of ADUs in census block groups was positively associated with street blocks that had alleys, the percentage of households that include relatives besides children under 18 or a spouse, and wealthier non-White households. Neighborhood wealth, residential unit density, and assessed value per square foot were all hypothesized to be positively associated, but were not found to be associated at all. This research provided a first look at a wide range of variables. Further research is warranted. Of particular interest is the potential effect of alleys in facilitating the provision of ADUs in single family areas.