Accessory Dwelling Units

Low-density residential neighborhoods around transit stations can accommodate significant numbers of new second units, as shown in the map of residential housing with potential space for second units within a half-mile radius of the North Berkeley BART station.

Accessory Dwelling Units

Karen Chapple, Associate Professor of City & Regional Planning


Jake Wegmann, Alison Nemirow, Colin Dentel-Post


Center for Community Innovation and University of California Transportation Center

Project Description

California’s implementation of SB 375, the Sustainable Communities and Climate Protection Act of 2008, is putting new pressure on communities to support infill and affordable housing development. As the San Francisco Bay Area adds over two million new residents by 2040, infilling the core (in targeted Priority Development Areas, or PDAs) could accommodate over half of the new population, according to the Association of Bay Area Governments (ABAG). But at the same time, infill could increase housing costs and exacerbate the region’s affordability crisis.

One potential solution is secondary units (also called in-law units or accessory dwelling units). Self-contained, smaller living units on the lot of a single-family home, secondary units can be either attached to the primary house, such as an above-the-garage unit or a basement unit, or detached (an independent cottage). Secondary units are particularly well-suited as an infill strategy for low-density residential areas because they offer hidden density, housing units not readily apparent from the street – and relatively less objectionable to the neighbors.

This study examines two puzzles that must be solved in order to scale up a secondary unit strategy: first, how can city regulations best enable their construction? And second, what is the market for secondary units? Because parking is such an important issue, we also examine the potential for secondary unit residents to rely on alternative transportation modes, particularly car share programs.

This report begins with an overview of demand for secondary units. Subsequent sections then describe the barriers—regulatory, market, and parking—to scaling up this housing strategy. After a discussion of the potential impact of a secondary unit approach, the report concludes with policy implications. Five working papers associated with this report (available via provide more extensive analysis and specific recommendations for each city.