Disclaimer:
All data should be considered estimates only. Estimates rely on a custom analysis of proprietary data performed by the Zillow Group, in consultation with reporters.

Data sources:
Property tax data courtesy of Zillow, which regularly collects data directly from county assessor offices. Assessed values and property taxes paid are from 2017 or 2018. Total property taxes paid include special assessments, parcel taxes and other property-based levies that exceed the 1 percent cap imposed by Prop. 13.

Home value estimates courtesy of Zillow’s Zestimate, which incorporates a variety of local market conditions to simulate housing values for properties not currently for sale. Single-family homes, duplexes, triplexes and condos are incorporated in this analysis, while apartment buildings are excluded. Home value estimates were taken from the year that matched the county property tax records. The number of housing units in each Census tract (excluding apartment units) was taken from the American Community Survey.

Missing data:
Zillow did not have property tax data for 18 California counties, most of which were low population counties. Census tracts where more than 40 percent of properties were missing reliable tax data or had suspect effective tax rates, and the top/bottom 2 percent of census tracts statewide by assessed and market value, were omitted. Roughly 10 percent of the state’s census tracts are missing from this analysis.

Methodology:
For each Census tract we excluded the top and bottom 10 percent of home values, assessed values, taxes paid and effective tax rate. We did this to limit the bias associated with outlier parcels, which might have unreliable tax or home value estimates. We calculated the mean for each of those values within each Census tract, and then multiplied that by the number of housing units in each tract.

Our Prop. 13 savings are calculated by subtracting the amount paid in property taxes in each Census tract (including parcel taxes and other assessments) from 1 percent of the total market value of homes in the Census tract. Our methodology likely underestimates savings from Prop. 13, as it tends to exclude the influence of higher-value homes. Negative values for some tracts indicate that properties taxed at 1 percent of market value would pay less than what they are currently paying. This is because of other property taxes beyond the one percent cap imposed by Prop. 13 and is more common among new developments.

Data sources and methodology:
Property tax data from Alameda County Assessor and Zillow. Property value estimates from Zillow. Both property tax and property value figures are from 2017, the most recent year reliable property tax data is available. Owner/renter designation derived from whether property homeowner claimed homeowner exemption. Estimated property taxes paid “without Prop. 13” were calculated by taking 1 percent of the 2017 Zillow market value estimate.

“Property taxes paid” include parcel taxes, special assessments and debt service taxes that cumulatively rise above the 1 percent cap imposed by Prop. 13. Property owners who would pay less in property taxes if taxed only on market value are excluded from overall revenue estimates.