In Santa Clara county your property tax is approximately 1.1% on the value of your property. In addition to that 1.1% you have assesments, those assessments will vary by area. Homeowners in Sunnyvale will have slightly different assesments than homeowners in Santa Clara.
When you purchase a home the seller must disclose the assesments the he or she pays. The value of your property has little to do with the with the value of the land or structure, but the price your paid for your home. If your value declines you can file a temporary Prop. 8 review however, that will not change whats refered to as your Prop. 13 value, that will always remain the same.