If you have experienced a reduction in the value of your property, yes, the Sonoma County Assessor's office will re-appraise it based on very recent closed sales data provided by you. The link below will take you to the Assessor's Office form to complete to apply for a property tax reduction. If you need help identifying three recently closed comparable sales (you'll see the place for this on the form), I'd be happy to talk with you and provide them. Or, you could contact the Realtor who helped you purchase your home as he or she would already by familiar with its features and could help you nail down good comps.
There are two parts to the answer. The first part is whether or not Sonoma County is doing the same as San Mateo in the excellent answer Artur provided. The short answer is that they have just started being more reasonable about reassessments. Until very recently they did not generally allow downward revisions of value and the appeals process was almost never successful. That is now changing.
The second part that you need to look at was hinted at in what Flaaash said. Values are generally dropping in the Russian River area, but not below values from four or five years ago. If you bought at the peak of the market in August of 2005, your home has probably dropped between 15% to 25%. If you bought before 2002 it is very unlikely that your home has dropped beneath the appraised value the tax assessor has established based on your purchase price plus annual 1% increase.
Larry Ellison, one of the richest people in the world (founder and CEO of Oracle) has just lowered his taxes. He argued that the property he spent $200 M to build a few years ago had become functionally obsolete and is worth $75 M at best now, if I can remember the number correctly. The San Mateo county assessment board agreed with him and lowered his taxes by $3 M per year. A lot of people complained that his move will lower local schools budget, as they are founded from collected real estate taxes. Nevertheless, the next day following the press announcement the local Assessor's phone is ringing of the hook as everyone is trying to do the same. So, good luck - obviously it is worth trying.
The trick is getting the county to take action. Many counties have complex apeals processes and it is difficult at best to get their attention on an individual property.
You should go to your assessors office and file an apeal of your properties proposed assessed valuation if you feel it is too high.
There are time constraints unique to all areas, so you will have to take the initiative here and do some digging.
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