Did California Overbuild its Housing Stock?

It all depends on the relevant time frame. If we look at the 00's in isolation, California overbuilt.

But California has grew by 7 million people between 1990 and 2015 and added about 2.43 million housing units (all data are from US Census-- assume that 98 percent of units permitted are actually built). The average household in California has 2.9 people (which is the second highest in the country, and compares with 2.5 nationally), which means that even without removals from the stock and no change in household size, the state needed 2.41 million new housing units. So if we look at the 18 year horizon, California did not overbuild--there we almost surely more than 20,000 demolitions over an 18 year period.

What if we go back to 1980? California grew by 13 million people and added 4.4 million housing units. At 2.91 people per unit, California demanded 4.46 million units. Again, it is safe to assume 60,000 demolitions over 28 years, so it is hard to make a case for long-run overbuilding.

Certainly, some housing was built in the wrong places, or was the wrong type of housing for the place (Lancaster and Beaumont come to mind). But it is hard to make a case that in aggregate California now has too many housing units.