Some properties (like you mentioned) are under the Williamson Act because the use is strictly for agriculture. The taxes are significantly lower but carry restrictions. If you decide to develop land like this for housing there is a long process involved in taking the property out of the Williamson Act. It is expensive and time consuming. There is no guarantee that you will be successful in changing part of it or all of it for residential use.
If you find a 10+ acre property along the Peninsula, it does not necessarily mean that it is used strictly for agriculture and carries the lower tax treatment. You can find more information on this at: http://www.conservation.ca.gov/DLRP/lca/Pages/Index.aspx
Mark Burns, Realtor
Coldwell Banker Elite - Top 1.5% Worldwide