On June 6, 1978, California voters overwhelmingly approved Proposition 13, a property tax limitation initiative. This amendment to California’s Constitution was the taxpayers’ collective response to dramatic increases in property taxes and a growing state revenue surplus. Proposition 13 (or "Prop. 13") rolled back most local real estate assessments to 1975 market value levels, limited the property tax rate to 1 percent plus the rate necessary to fund local voter-approved bonded indebtedness, and limited future property tax increases to a maximum of 2% per year.

Prior to 1978, real property was appraised cyclically, with no more than a five-year interval between reassessments. Since property values were systematically reviewed and updated, assessed values were usually kept at or near current market value levels. In contrast, under Proposition 13, properties are reassessed to current market value only upon a change in ownership or completion of new construction (called the base year value). In addition, Proposition 13 generally limits annual increases in the base year value of real property to no more than 2 percent, except when property changes ownership or undergoes new construction. Essentially, Proposition 13 converted the market value-based property tax system to an acquisition value-based system.

Under Proposition 13, similar properties can have substantially different assessed values based solely on the dates the properties were purchased. Disparities result wherever significant appreciation in property values has occurred over time. Longtime property owners, whose assessed values generally may not be increased more than 2 percent per year, tend to have markedly lower tax liability than recent purchasers, whose assessed values tend to approximate market levels.