Accessory dwelling units — better known as cottages, in-law apartments or granny flats — could provide an estimated 1,000 new residences near selected BART stations, research by UC Berkeley Professor Karen Chapple shows.
ADUs diversify and increase the housing stock without enlarging a neighborhood's footprint, while allowing senior citizens to find a smaller dwelling without leaving their neighborhood, or college graduates to afford of a modest room of their own. With Bay Area housing in perpetual short supply, ADUs could provide a much-needed supply-side boost.
The catch? Existing zoning laws in the five-city study area of Albany, Richmond, El Cerrito, Berkeley and Oakland. Minimum lot sizes disqualify small properties from building ADUs, lot-coverage maximums and property-setback minimums restrict ADU placement and architectural design, and parking requirements—well, these stop many projects before they’re even started. Several cities require an additional unit of off-street parking for an added dwelling unit but do not allow tandem, or end-to-end, parking. This effectively requires an ADU builder to laterally extend a driveway (read: pave over part of the front yard) in order to provide a third independently accessible parking space.
The tale of the tape is that out of approximately 5,400 single-family residences within a half-mile radius of El Cerrito del Norte, El Cerrito Plaza, North Berkeley and Ashby BART stations, only 1,460 are even eligible for ADUs under current zoning laws. Eliminating the Berkeley lot size minimum and reducing the El Cerrito setback distances and parking requirement would increase that number to 3,568 eligible homes.
Chapple’s market research indicates that 28 percent of single-family homeowners in station areas have considered building an ADU; that's about 1,000 homes. While many in that tally face barriers under existing zoning laws, simplifying and reforming the code would allow the market to decide how many units will be built.
Widening the analysis to El Cerrito and Berkeley at large shows that reducing the zoning requirements doubles the properties eligible for ADUs from 4,220 to over 8,400. Assuming a market interest of 28 percent, and including the already-analyzed station areas, zoning reform could generate an additional 2,300 units of housing.
And that 28 percent figure is hardly fixed: as more baby boomers age, more will look for smaller, single-story housing within their neighborhood.