Expanding Affordable Middle Housing Options in Single-Family Neighborhoods

Problem, research strategy, and findings By examining the Residential Infill Project (RIP) in Portland (OR), this study addressed the ongoing debate on single-family zoning reform, focusing on the effectiveness of legalizing middle housing in expanding housing options in single-family neighborhoods. The analysis revealed a substantial increase in the proportion of middle housing in residential developments in single-family zones, from 13.4% before the RIP implementation to 44.7% in the 2 years after RIP's adoption. Almost 40% of these post-RIP middle housing permit units were replacements for existing homes. Compared with single-family housing developments, middle housing projects were more likely to be in neighborhoods with greater density, better walkability, and lower income. The RIP broadened middle housing accessibility by allowing their development on non-corner lots, at greater distances to the city center, and in medium-density single-family zones. The RIP's approach to regulating development density and building size significantly reduced the average size of middle housing units in single-family zones to less than half that of single-family homes. Given the strong correlation between home sizes and prices, it seems reasonable to infer that middle housing units have become considerably more affordable since the enactment of the RIP.