Proposition 90 is a law that allows for the transfer of base year value (in regards to property tax assessments) from any county in California to a county that has adopted the ordinance. El Dorado County supervisors approved the ordinance the end of 2009 and it went into effect in El Dorado County February 15, 2010. On January 27th the El Dorado County Board of Supervisors voted to extend it until October 1, 2016. In a nut shell, a homeowner in California can sell their home, buy a replacement property in a participating county, and take their lower tax base to the new county for assessment purposes.
There are some restrictions, however besides it being a participating county which includes that the claimant or claimant's spouse must be at least 55 years of age unless severely and permanently disabled in which there is no age restriction. There are a few other restrictions as well but if you qualify, it could be a big savings for you on your property taxes. You can read all the details about it on the El Dorado County Tax Assessors web site.
Following are other California counties that are currently also allowing Prop 90 tax base transfers : Alameda, Orange, San Mateo, Ventura, Los Angeles, San Diego, and Santa Clara, San Bernardino. Because the participating counties are subject to change, it is recommended that you contact the county to which you wish to move to verify your eligibility.
More Information on Proposition 90 including an example on how it works.