A Prop 8 decline in value review is how a California owner asks the county assessor to lower an assessment when the property's market value on January 1 falls below its Proposition 13 value. The reduction is temporary, the assessor reviews it every year, and the value can't rise above the Proposition 13 figure this way.
This guide explains how the review works, what evidence counts, and the deadlines we confirmed for Santa Clara, San Mateo, Alameda and San Francisco counties, using State Board of Equalization (BOE) and county pages read in November 2026. A supplemental bill after a purchase follows different rules, which our guide to supplemental property tax in California covers.
What Proposition 8 does
Proposition 13, approved in 1978, set a base year value for each property: its 1975 market value, or its value when it last changed ownership or was newly built. Each year that value can rise by the California Consumer Price Index change or 2 percent, whichever is lower, and the result is called the factored base year value. In November 1978, voters passed Proposition 8, which allows temporary reductions when a property suffers a decline in value. The BOE's decline in value page says it's codified in Revenue and Taxation Code section 51(a)(2).
On the January 1 lien date, the assessor enrolls the lesser of the factored base year value or the market value. The BOE adds two points that matter:
It's property by property. Even when the overall market falls, a property doesn't get a lower assessment unless its own market value drops below its factored base year value.
Several things can cause it. The BOE says a decline may come from changes in the real estate market, the neighborhood or the property itself. Its example is an owner who bought before the market fell dramatically.
Decades of ownership can leave a factored base year value far below market value, and then a dip in prices may not reach it. Recent buyers who paid near a high point fit the BOE's example.
Step one: the informal review with the assessor
The BOE says county assessors generally will review an assessment for a possible decline in value if the owner asks. Some handle it as an informal conversation with staff; others require a request form. The typical sequence the BOE describes:
Ask the assessor for a review, on a form or informally depending on the county, and give your opinion of value with supporting information, usually comparable sales close to January 1 but no later than March 31.
Submit by the county's deadline.
An appraiser reviews it and analyzes market value as of January 1.
If market value is lower than the factored base year value, the lower value is enrolled.
The assessor notifies you of the value.
Some assessors also review values without a request. Alameda County's assessor says it reviews values proactively, so an owner may not need to ask, and that once a property's value has been reduced it must be reviewed every following January 1.
An informal review is not an appeal. San Mateo County's assessor says the request shouldn't be confused with the more formal filing of an assessment appeal. The BOE's FAQ says that if you haven't heard back before your county's appeal deadline, you can file a formal appeal with the clerk of the board to protect your right to a review.
Deadlines in four Bay Area counties, as of November 2026
Formal appeals of the regular roll start July 2. The BOE's June 2, 2026 letter to assessors No. 2026/023 says the period ends September 15 where the assessor mails assessment notices to all owners on the secured roll by August 1, and November 30 where it doesn't, under Revenue and Taxation Code section 1603. Its 2026 county list and the counties' own pages show:
Santa Clara County: September 15, 2026 (now closed). The assessor's Prop 8 page says value notices go out the last week of June, its informal review filing period runs June 30 to August 8, and results are sent shortly after August 15. Whatever the result, you can still file a formal appeal, which the page says can take up to two years to resolve.
San Mateo County: November 30, 2026. As of November 2026, the assessor's page says it's no longer accepting new informal applications for the 2026-27 tax year and will accept applications for the January 1, 2027 value starting January 1, 2027. Formal appeals for 2026 can still be filed through November 30, 2026.
Alameda County: September 15, 2026 (now closed). The assessor's page, written for the 2025-26 roll, accepts informal requests for the current or upcoming year, offers an online form that needs at least one comparable sale, and asks for sales no later than March 31.
San Francisco: September 15, 2026 (now closed). The Assessor-Recorder's free informal review runs January 2 through March 31 for single-family dwellings, condominiums, townhouses, live-work lofts and co-op units, and it doesn't accept informal filings by third parties. Results arrive in July on the Notice of Assessed Value. A formal appeal carries a non-refundable $120 filing fee.
The BOE's 2026 list also puts Contra Costa, Marin, Napa, Santa Cruz, Solano and Sonoma counties on the November 30 schedule.
Evidence that counts
The BOE's February 2020 appeals guide, Publication 30, says sales of similar properties are generally the strongest evidence in a residential appeal. It judges a comparable sale three ways: was it an arm's-length, open-market sale; is the house physically similar; and is the sale close enough to the valuation date? For a decline-in-value appeal, the valuation date is January 1 of the year you're appealing, and the guide says an appeals board can consider only sales that closed no later than 90 days after it. It names later sales as the leading error in assessment appeals.
Physical similarity includes distance, zoning, bedrooms and bathrooms, year built, house size, lot size and view, extras such as pools, quality of construction and condition. That's hard with a post-and-beam house whose nearest sale may be a very different design, and our guide to pricing an architectural home when the comparables don't match walks through the method.
A few other points from the same guide:
Check the assessor's data on your own house (age, building size, lot size) before you gather sales.
Appeals boards can't lower a value just because you pay more tax than a neighbor, so a neighbor's lower assessment isn't evidence of market value.
Many assessors keep sales lists you can inspect for little or no cost.
If you file a formal appeal
A formal appeal goes to the county assessment appeals board on the county's own Assessment Appeal Application. Publication 30 describes how it works as of its 2020 edition:
One year at a time. A decline-in-value appeal covers only the year you file. You file again for each year you disagree, even if a prior year is still pending.
The board can go either way. It can lower, raise or confirm the value.
Burden of proof. For an owner-occupied single-family dwelling that is your principal residence, the assessor presents evidence first and bears the burden of proof.
Timing. You get at least 45 days' notice of the hearing. Boards are expected to decide within two years of filing, and if they don't, your opinion of value may temporarily become the taxable value, with exceptions.
Keep paying. Taxes are due on time while an appeal is pending; a reduction brings a refund with interest.
The BOE's FAQ adds that Proposition 8 doesn't apply to supplemental assessments. If you think the value set after your purchase is wrong, that's a base year appeal, due within 60 days of the supplemental notice under Publication 30, or in a regular filing period in the year the value first goes on the roll or the three years after.
After a reduction, and when damage is the cause
A reduced value doesn't freeze. The BOE says properties in decline-in-value status are reviewed each year and can rise by more than 2 percent, but never above the factored base year value unless there's a change in ownership or new construction. Its FAQ adds that once a property has been in this status for two or more years, the year-to-year change follows the market in either direction.
If a fire, flood or other calamity damaged the house, the BOE describes a separate route under Revenue and Taxation Code section 170. The loss must be at least $10,000 of current market value, the claim is due within 12 months of the damage or the county's longer period, and the BOE says all California counties have adopted an ordinance for it. A home rebuilt in a like or similar manner keeps its prior base year value. Publication 30 gives six months from the notice to appeal a calamity reassessment.
Heirs keeping a parent's Proposition 13 value face a different set of rules, covered in our Prop 19 inherited home guide.
This is general information as of November 2026, not legal or tax advice. Filing windows, forms and fees vary by county and change each year, so confirm the details with your county assessor and clerk of the board, and talk with a tax advisor or attorney about your own property.
Frequently asked questions
Can I ask for a Prop 8 reduction for past years?
No. The BOE says only the latest January 1 assessment can be reviewed, and Proposition 8 gives no relief for prior years. If you missed a year, focus on the next January 1 value.
Do I keep paying my full tax bill while a request is pending?
Yes. The BOE's FAQ says you pay according to your bill, or penalties and interest apply. If the assessor grants a reduction, it processes a correction and a revised bill or a refund.
Do I need to hire someone to file?
No. Publication 30 says you aren't required to have an attorney or agent at an appeal hearing, and San Francisco's informal review is free. Someone filing for you needs your written authorization unless that person is a California attorney or your spouse, parent, child or registered domestic partner.
Related reading
Mills Act in California: Historic Designation and Mid-Century Homes
Selling a Mid-Century Modern Home: What to Fix and What to Leave
Prop 19 Base Year Transfer: 55+, Disabled and Disaster Moves
California Homeowners' Exemption: How to Claim It After You Buy
Property Tax Due Dates in California: Paying a Bay Area Tax Bill
Does Remodeling Increase Property Taxes in California? What Counts
More guides on this topic: Buying & Selling a Mid-Century Modern Home
Bought a mid-century home recently and wondering how its value compares with your assessment? Call or text the Boyenga Team at Compass at (408) 373-1660 or (650) 383-8606, or contact the Boyenga Team to talk through recent sales before the next January 1 lien date.
