If you believe that your property has been inaccurately assessed, you can file an appeal. The appeal window for all regular appeals in all counties opens July 2nd for commercial and residential properties and the filing deadline varies by county. Most counties are either September 15 or November 30, and you can check with your local county tax assessor’s website or see your valuation notice for more information.

In the case of a Supplemental assessments, the deadline to appeal is within 60 days of mailing date or postmark date on the notice, whichever is later.

Note that there is a filing fee for some counties in California, including Contra Costa and Los Angeles county.

There are two types of appeals. The first type of appeal is called "Decline In Value". Proposition 8 allows all property owners to request a property valuation review from their county assessor. This process involves filing the appropriate forms and compelling evidence with the county assessor’s to reduce a property’s assessed value. The local assessor will review and respond to these requests on a rolling basis, typically prior to any formal appeal process.

The second type of appeal is called a "Base Value Appeal". You can only appeal your property’s base value within 3 years of when it is set either by a purchase or new construction. When appealing the base value of your property, you are arguing the value that sets the basis from which your property tax assessment will grow for years to come. Base value appeals can be quite valuable when contemplating the multiple years of tax savings.

Every county in California has a process by which you may contest or appeal your property’s assessed value. The formal process of appealing your property valuation often takes many months.