Price curves are very nonlinear, especially surrounding the point where supply = demand. Housing prices in the Bay Area have about doubled over the last 10 years, while the population has increased by about 10%.

Anyhow, the point of all of this legislation isn’t that duplexes alone will necessarily provide enough housing. It just gives cities a fair chance to meet their housing targets via duplex building and preserve some local zoning preferences. If they fail, the state will step in to void local zoning laws and approve dense apartment buildings.