Property tax assessment in California is a system built on a unique constitutional framework that shapes how every property owner interacts with local government. At the center of this system is the county assessor, an elected or appointed official responsible for determining the taxable value of real property within the county. The assessor does not set tax rates; instead, the assessor establishes assessed values, and those values are multiplied by the tax rate to produce the annual property tax bill. Understanding how assessment works begins with understanding the assessor’s role and the rules that govern valuation.

Every piece of property in California is tied to an assessor parcel number, often called an APN. This number is a unique identifier assigned by the county assessor to each parcel of land for tax purposes. The APN typically appears on property tax bills, deeds, and recorded documents, and it allows the assessor, the tax collector, and the recorder to track a specific property through the system. Because street addresses can change and multiple parcels can share an address, the APN is the most reliable way to identify a property in county records. Property owners should know their APN, since it is required for many transactions, including paying taxes, filing assessment appeals, and researching a property’s history.

The most important feature of California property tax assessment is the base year value established under Proposition 13, the landmark 1978 ballot measure. Proposition 13 generally limits the assessed value of a property to its base year value, which is the market value at the time of the most recent change in ownership or new construction. For properties that have not changed hands since 1978, the base year value reflects the 1975 assessment plus an adjustment for inflation. Each year, the base year value may be increased by an inflation factor, but that increase is capped at two percent annually, regardless of how much the market value of the property has risen.

This base year system creates significant differences between the assessed value and the actual market value of a property. A home purchased decades ago may carry an assessed value far below what it would sell for today, while a recently sold home will be reassessed at its purchase price. When a property changes ownership, the assessor establishes a new base year value, generally equal to the purchase price. New construction is treated similarly, with the value of the improvements added to the assessment. Certain exclusions and transfers between family members can allow a base year value to carry over, which is why Proposition 13 has become a subject of ongoing policy debate.

Property owners who believe their assessed value is incorrect may file an assessment appeal with the county assessment appeals board. Grounds for appeal typically include errors in the assessor’s valuation or a decline in market value below the factored base year value. The assessor parcel number is essential for filing an appeal and for any correspondence with the county. Together, the assessor parcel number, the assessor’s valuation practices, and the Proposition 13 base year rules form the foundation of California’s property tax assessment system, a structure that continues to shape property ownership and local government finance across the state.

For more on this, see how assessment works.