Supplemental Tax Estimator - Office of the Assessor - Alameda County

Estimator inputs

Purchase Date (use the calendar or type the date)

Purchase Price or Fair Market Value

Minus Roll Value

Instructions

To use the Supplemental Tax Estimator, please follow these instructions:

Enter a purchase date no earlier than 01/01/ and no later than 12/31/ of the applicable assessment year. Use the date picker (calendar) or type the date in mm/dd/yyyy order as shown in the field (for example enter 05/15/2025 for May 15, 2025).
Enter the purchase price (or fair market value) using numbers only.
Enter the roll value or your best estimate in the “Minus Roll Value” field.
To find the current roll value, use the Find Roll Value link next to the “Minus Roll Value” field. Enter the parcel number or the property address and then click Submit.
The Total Taxable Value shown in the table is the most recently surrendered roll value unless affected by other atypical situations as previously referenced. Return to the Estimator by clicking the Back button on the browser and enter the roll value.
Once all of the information is entered into the appropriate fields, click the “Get Estimate” button.

When real property is purchased, a supplemental assessment is imposed from the date of purchase pro-rated through the current tax year. The Supplemental Tax Estimator provides an estimate of how supplemental assessments are calculated along with the resulting estimated supplemental taxes. Please note that the Estimator is intended for changes in ownership only and not for situations where assessed value is added due to new construction.

The Supplemental Tax Estimator will provide estimates for all types of properties; however the results are most reliable for single-family residences and condominiums. Sales prices on these types of properties typically reflect the market value of the property. Change in ownership situations without a sales price (inheritance, foreclosure, etc.) will require an estimate of the fair market value of the property. The accuracy of the supplemental tax estimate will be dependent on the relationship of the purchase price to fair market value.

Situations when the Estimator may not be reliable

There are a number of situations in which the Supplemental Tax Estimator will not be able to produce a reliable estimate. The most common are:

More than one change in ownership has occurred in a calendar year.
Assessed value is added due to new construction.
A change in ownership and assessable new construction has occurred in a calendar year.
Partial interest transfers where less than 100% of the property has changed ownership.
Some new housing tracts or condominium subdivisions in which the map recording and change in ownership has occurred in the same calendar year.

Note that if the purchase date is between January 1 and May 31, there are two supplemental assessments. The first supplemental (not calculated using the Estimator) is the difference between the prior year roll value and the newly established fair market value. The second supplemental, as calculated by the Estimator, which is the difference between the current roll value and the purchase price (or fair market value). If the purchase date is between June 1 and December 31, there is one supplemental assessment, as calculated by the Estimator, is the difference between the current roll value and the purchase price (or fair market value).

If you have atypical situations or have additional questions about supplemental assessments, please call the Assessor’s Office at (510) 272-3787.