Prop 8 Decline-in-Value: How to Lower a California Assessment

California must lower your assessment, not just may, whenever your home's market value on the January 1 lien date is less than its Proposition 13 factored base year value. This is Proposition 8, codified at Cal. Rev. & Tax. Code section 51(a)(2). You do not need to wait for the regular July through September or November filing window to start this process: you can ask your assessor for an informal review at any time, and if you disagree with the result you can still file a formal appeal within your county's regular period. Either way, the reduction only lasts as long as the market stays down, because the assessor reviews it every year.

Two values, and the lower one wins

Under Proposition 13, your home has a base year value set when you bought it or built on it, and that value is "factored" upward each year by the lower of the change in the California Consumer Price Index or 2 percent, according to the Board of Equalization. In a normal year, that factored base year value is also your assessed value, because it usually sits below current market value.

Proposition 8 changes that when the market falls. The Board of Equalization's own page states it plainly: "A decline in value occurs in any year in which the current market value of real property is less than its adjusted base year value as of the lien date, January 1." When that happens, "the Assessor will review the property's assessment and enroll the lesser of the factored base year value or market value." So the rule is not that your Prop 13 value goes away. It is that the assessor has to compare the two numbers every year and use whichever is lower.

Why January 1 is the date that matters

Both values are compared as of the lien date, January 1, not any other day of the year. If your home was worth less than its factored base year value on January 1 but has since recovered, the comparison the assessor makes still uses the January 1 figure for that tax year. The reverse is also true: a temporary dip in the spring or fall that has passed by the following January 1 will not show up in the comparison at all, because the assessor is not tracking your home's value continuously. It is a single snapshot, taken once a year.

This is also why the timing of your evidence matters. Sales, listings, and appraisals dated far from January 1 are weaker evidence of what your home was worth on that specific date.

The informal review vs. the formal BOE-305-AH appeal

There are two separate paths, and they are not the same thing.

The informal path is a conversation. Board of Equalization Publication 30 recommends it as the first step: "Before filing an application, you should first meet informally with the assessor to discuss the assessment." You can raise a decline-in-value concern this way at any point, and many assessors proactively review property values for a decline each year without you asking. If the assessor agrees your market value fell below your factored base year value, they adjust the roll and you never need a formal filing.

The formal path is the application, form BOE-305-AH, filed with the clerk of the county board, which acts as the Assessment Appeals Board. You need this path if the informal review does not resolve the disagreement, or if you want a decision you can enforce rather than a discretionary adjustment. A formal appeal still has to be filed inside your county's regular window, either September 15 or November 30 for 2026 depending on the county; the California deadline guide has the date for all 58 counties. An agent can file and argue the formal appeal for you if you provide written authorization, per Publication 30.

Do both if you are not confident the informal conversation will resolve it. Publication 30's guidance to talk to the assessor first does not extend your filing deadline, so if the window is closing, file the formal application and keep talking informally in parallel.

The 90-day comparable sales rule

Evidence matters more in a decline-in-value case because you are arguing a specific number: what your home was worth on a specific January 1. Publication 30 sets a rule for how recent your comparables need to be: for a decline-in-value case, comparable sales should be no later than 90 days after the January 1 lien date. A sale from March or earlier in the same year as the lien date is useful. A sale from August is outside the window Publication 30 describes, and sales from the prior year are further still.

Picking and adjusting the right comparables is its own skill. The guide to finding comparable sales covers which sales count, how to adjust for size and condition differences, and how to present them so an assessor or hearing officer takes them seriously.

The reduction is temporary, reviewed every year

A Prop 8 reduction is not a new permanent base year value. The Board of Equalization is explicit: "Once a property's assessment has been reduced under Proposition 8, the Assessor reviews the assessment annually to determine whether it should remain in decline-in-value status." The reduction is temporary, and the value can rise back toward the factored base year value as the market recovers.

That cuts both ways. You do not typically need to re-file every single year to keep a decline-in-value reduction; the annual review is the assessor's own job. But you also should not expect the lower number to last once home prices in your area recover. If your factored base year value is $600,000 and your Prop 8 value drops to $500,000 this year, next year's comparison starts over: if the market has recovered to $580,000, that is what gets enrolled, still below the factored base year value but a real increase from the prior year.

A worked example (illustrative)

This example is illustrative only. The numbers are assumptions to show how the comparison works, not a real case or a typical outcome.

Assume a home has a Prop 13 factored base year value of $620,000 as of January 1, 2026, after growing at roughly 2 percent a year since it was purchased. Assume its actual market value on that same January 1 has fallen to $550,000, based on nearby comparable sales dated within 90 days of the lien date. Because $550,000 is less than $620,000, section 51(a)(2) requires the assessor to enroll the lower figure, $550,000, for the 2026 tax year.

Assume further, for illustration only, an effective property tax rate of 1.1 percent. At the factored base year value, the bill would be about $6,820. At the Prop 8 value, it would be about $6,050, a difference of roughly $770 for that year. Because the reduction is temporary, next year's bill depends on where the market and the factored base year value stand as of the next January 1, not on this year's number.

If your own numbers look like this, comparing your assessed value against real sales is the place to start; a free review of your address is a way to get that comparison done without doing the research yourself.

Common questions

Does a Prop 8 reduction permanently lower my Prop 13 base year value?

No. Your factored base year value keeps growing in the background at the same rate it always would, capped at the lower of the CCPI change or 2 percent a year. Prop 8 only affects which of the two numbers, factored base year value or current market value, gets enrolled for a given tax year.

Do I have to reapply every year to keep a decline-in-value reduction?

Not necessarily. The Board of Equalization's own description says the assessor reviews decline-in-value status annually as part of the regular process. But nothing stops you from requesting a review yourself if you think the assessor missed a continuing decline.

What if the informal review doesn't lower my value enough?

File the formal application, form BOE-305-AH, with your county's clerk of the board within the regular filing period. The California deadline guide has each county's 2026 closing date.

Can I use any sale from the past year as a comparable?

Not for a decline-in-value case specifically. Publication 30 states comparable sales should be no later than 90 days after the January 1 lien date. The comparable sales guide covers how to choose and adjust sales within that window.

Is a Prop 8 review the same as the full appeal process described in the general guide?

It is one specific argument, not the whole process. The complete guide to appealing your property taxes covers the full process, including record errors, deadlines, and the hearing, in case your situation is not only a decline in market value.