Planners are encouraging suburban homeowners to build second units to enable housing affordability, walkability, and aging in place. Yet, little is known about the viability of this approach for different types of suburbs. Based on planner interviews and zoning ordinance reviews in the Phoenix region, this article constructs a typology for identifying suburbs potentially amenable to using second units as a tool. Barriers that limit rezoning to allow for second units across the types include restrictive site requirements and fear of rental units. Arguments for aging in place may be more influential than those for housing affordability in leading suburbs in Phoenix and elsewhere to liberalize their regulations.