See the rules regarding "minimum zoning" applying to PDAs - Priority Development Areas defined by ABAG - (friends of mine happen to live in one).
Say you or your neighbor's house burns down...too bad you can't rebuild it or finance it due to the minimum zoning regulation (unless you are granted special dispensation).
The rule: either rebuild an apartment, or sell up to an apartment developer (likely at a marginal profit) and move elsewhere!
The details are here:
http://citizenmarin.org/priority-development-areas/
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