Below is a chart of residential vacancy rate by state from the 2010 US Census.
(click to enlarge)
California has the second lower vacancy rate among the 50 states and DC. California builders may have built the wrong kind of housing in the wrong places, but overall, they did not build too many houses.
Note that Florida and Arizona have very high rates, but their rates are always high, because so much of their housing stock is seasonal–I suspect vacation homes drive a lot of what is happening in Maine, Vermont and Alaska as well. Nevada has had the largest increase in vacancy over the past 20 years, rising from about 10 percent to 14 percent.