What does Revenue and Taxation Code section 51 say about my California assessment?

Cal. Rev. & Tax. Code §51 is the statute that operationalizes Proposition 13's value cap together with Proposition 8's relief. It states that for each lien date after the base year is set, the taxable value of real property is the lesser of:

1. its factored base year value — the Prop 13 base value adjusted by the annual inflation factor (capped at 2% per year), or 2. its full cash value (current market value) as of the January 1 lien date.

This lesser-of rule is the single most important concept for a California appeal:

• It protects you in a rising market. Even if your home's market value jumps, you are taxed on the lower factored base year value — there is usually nothing to appeal.
• It entitles you to relief in a falling market. When market value drops below the factored base year value, §51 requires the assessor to enroll the lower market figure — this is the Proposition 8 decline-in-value reduction described on the state BOE decline-in-value page.
• It defines what you must prove. To win a Prop 8 appeal you must show, with comparable sales near the lien date, that current market value is below the factored base year value the assessor enrolled.

Section 51 also explains why Prop 8 reductions are temporary: once market value recovers, the assessor restores the enrolled value year over year, never exceeding the factored base year value ceiling. The annual factor itself comes from the change in the California Consumer Price Index but is statutorily limited to 2%. Understanding §51 tells you whether you even have a case — if your factored base year value is already well below market, an appeal will not help and could expose you to an upward adjustment under §1610.8.