If you go to the tax assessor's web site (http://www.sccassessor.org) you will find a form to complete. You will need to support it with like homes that have sold within one mile of your home in the last 12 months. They are looking for a change in value tax-year to tax-year. Have an agent that works in your neighborhood assist in pulling comps. I have done this before for folks.
Make sure to click the "more information" link near the bottom of the page it outlines how your application will be reviewed:
Per Proposition 8, you need to turn in a Decline in Value Request
or you can contact the:
Division Real Property
For More Information Please Contact:
Real Property Phone
Phone: 408-299-5300 (Property valuations, Propositions 3, 60, 90)
Phone: 408-299-5350 (Williamson Act)
Fax: 408-298-9439 (for above services)
Phone: 408-299-5371 (Possessory Interest)
Also, you need to ask your agent to give you comps for a year and turn it in before the deadline.
To apply for a reduction for the 2012-13 Assessment Roll please visit the Assessor's website after you receive your notification card in June 2012.
The values are
At the Santa Clara County Assessor's site http://(www.sccassessor.org/prop8) you will find a form to complete.
Enlist the help of a neighborhood specialist in providing the supporting data of similar closed homes within one mile of your home in the last 12 months. Keep in mind an appeal will be rejected if the request is based on only 1 or 2 low-priced homes when the others are all selling at your current assessed value or higher.
You can locate the specific form here:
For Cupertino if the price drops 1-3% for the same size, and you are forcing the issue by contacting the office
you may get an adjustment. From what I recall once you challenged the authority your property will quickly get adjusted and watched for a re-assessment in an up swing. That happens to one of my clients.
As for collecting comp data it is better to get a State Licensed Appraisal who does it correctly and is qualified to represented you.