The California population was increasing by 18% to 50% per decade from 1940-1980 compared to US population growth of 7-18%. Lots of people wanting to move to California means you need to make houses quickly for them to live!
Skipping basements speeds up the house building process quite a bit. Builders don’t need to dig up dirt and move it elsewhere and concrete is only necessary under the exterior walls for a crawl space foundation. California also has/had an excess of wood for everything above the ground (sorry ancient redwood forests).
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