This paper begins by analyzing how many secondary units of one particular type, detached backyard cottages, might be built in the East Bay, focusing on the Flatlands portions of Berkeley, El Cerrito, and Oakland. We then investigate the potential impacts of scaling up the strategy with regard to housing affordability, smart growth, alternative transportation, the economy, and city budgets. A final section details policy recommendations, focusing on regulatory reforms and other actions cities can take to encourage secondary unit construction, such as promoting carsharing programs, educating residents, and providing access to finance.