University of California Transportation Center

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 two million new residents by 2035, 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.