That U.S. Census Bureau data says so probably doesn’t surprise you.
However, the data pokes holes in the narrative that Silicon Valley transplants are the main reason for the Puget Sound’s explosive growth in population—and housing costs.
More of the out-of-state newcomers to King County from 2012-16 were from California than the rest of the top five states combined. Eight of the twenty out of state counties that sent the most people to Seattle are in California.
However, more of the people new to King County were nearby already. Most of the people who moved to King County in the same period already lived in Washington, and the Puget Sound region itself. Together, Pierce and Snohomish Counties sent more people to King County than California.
Also, Southern California sent more new residents to King County than the Bay Area, and Los Angeles by itself sent nearly as much. The five Bay Area counties (San Francisco, Santa Clara, Alameda, San Mateo, and Contra Costa) sent 5,158 people. Los Angeles County sent 4,550. The Southern California counties (Los Angeles, San Diego, Orange, and San Bernardino) sent 8,542.
The Census Bureau estimates that 143,088 total new residents moved to King County between 2012 and 2016. That number combines people moving from foreign countries, people moving from other U.S. states, and Washington itself.
The data comes from U.S. Census Bureau figures compiled from 2012-16’s American Community Survey (ACS) statistics, which estimate county-to-county migration in each county in the United States. These figures measure only in-migration—the number of people who moved to King County—not net migration, which would account for the combined in- and out-migration.