Modeling Upzoning's Impact in the Bay Area

California Community Builders (CCB) is a California policy advocacy organization working to close the racial wealth gap through housing and homeownership reform. To build the evidence base California's zoning reform debate had been missing, CCB partnered with HR&A and HousingWeaver to build a parcel-level econometric model estimating how modest upzoning — allowing 2- to 4-unit buildings in single-family zones — could affect housing supply and affordability across six Bay Area cities.

The Challenge

Debates over upzoning in California had grown lengthy and contentious, with supporters and opponents trading competing claims about its effects on housing supply, affordability, and displacement. In the wake of multiple defeated housing bills and a housing crisis deepened by the COVID-19 pandemic, CCB needed rigorous, place-based analysis to quantify the impacts of modest upzoning.

The Approach

Using HousingWeaver, HR&A built a parcel-level model across six Bay Area cities — Los Altos, Saratoga, Pleasanton, Milpitas, Antioch, and Dublin — selected to represent a wide range of market conditions.

For each parcel, the model:

• Accounts for who owns the property, local land and construction costs, and how likely a given lot is to change hands in any given year
• Tests whether a developer could turn a profit by building new units on the site
• Estimates how many units each redeveloped site would yield and what those units would cost, relative to the homes they replace
• Projects the income a buyer would need to purchase in each city, revealing who the new supply would likely reach across vastly different markets

Results & What's Next

The findings were consistent across all six cities. Modest upzoning would produce a 14% increase in housing supply over five years, with fewer than 2% of single-family homes redeveloped in any given year. Each redeveloped parcel yielded an average of 3.25 new units at prices well below the homes they replace: in Los Altos, the income required to buy drops from approximately $630,000 to $200,000; in Milpitas, to $120,000; in Antioch, to $70,000. The analysis continues to ground CCB's advocacy in California and was published in The Effects of Upzoning Single-Family Neighborhoods on Supply and Affordability. The underlying methodology can be adapted for other states and cities where the same debate is underway.