It’s been about 10 years since Contra Costa County reduced property taxes on a large scale. Proposition 8 passed by California voters in 1978, amended Prop 13 to allow for a temporary tax reduction when a property’s market value as of January 1 of each year is below the “base-year” value. The base year value is typically your purchase price with a possible increase for inflation which should not exceed 2% per year. Permitted additions and renovations to you home do add to the tax base too.
Here is the link to the actual instructions to apply for the tax reduction in Contra Costa County,
Remember the assessor’s office will look at the value as of January 1, so you may want to wait until next year. The strongest comparatives in your neighborhood are within 90 days.
By the way, even if you request a review, you still must pay your property tax bill in full on time to avoid penalties.