What is the end solution to this housing crisis? The solutions will all hurt existing homeowners or multi-homeowners and are not likely to be implemented. The duplex in a single family lot is not going to move the needle on this crisis much, as building 2 houses on a existing lot will require a teardown of the original structure, then a buildup of the new duplex, all very costly. It will run at least $500K+ and I don't see many middle class owners using this route, also construction workers are scarce in the state due to high housing costs.
What would truly fix things right away:
#1. Prohibit Companies/Entities/hedgefunds from buying houses and renting them, ala zillow, blackrock etc.
#2. Repeal CA prop. 13, all housing should be taxed at current market value. (exclusions for older retirees and the disabled)
#3. Homeowners that wish to own a second property should be taxed at a very high rate.
#4. Mandate real estate commissions to be halved, currently its 6%(3% buyer, 3% seller) it should be closer to 3% total, this would allow greater mobility and encourage more buying and selling of houses.
#5. Rezone unused commercial(3 years unused) to residential to allow for more units to be built.
None of these solutions above would change the makeup of a neighborhood by introducing high density housing which some owners don't want, and could easily be implemented by a few laws being crafted and passed. I bet 10-15% of the housing market would free up right away and prices would come down 20-30% within a year or two.
But the root of the problem is just the people who are intentionally moving to southern California due to the mild climate. They know how crowded and dry[lack of water](and also expensive) it is but move here anyways contributing to the congestion. They don't want to move to other areas even when there are jobs there, and this issue will always be there.