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Frequently Asked Questions

The following are questions we often receive about supplemental property taxes. For specific information, please call us at (650) 363-4142 or visit our office.


What are "supplemental" property taxes?

"Supplemental" taxes are additional secured taxes that are due when property undergoes a change in ownership or new construction. The additional tax is owed because the County Assessor is required to immediately adjust the January 1 value to reflect the new value of the property (see Secured Tax definition). The supplemental tax bill represents the tax due on the difference between the old and new values adjusted for the number of months left in the fiscal year.

When is the supplemental tax due?

The supplemental tax is due upon the mailing of the bill. It is payable in two installments.

When does the supplemental bill become delinquent?

The supplemental bill is payable in two installments, like the secured bill. The delinquent dates of the installments depend on when the bill is mailed. A supplemental tax bill mailed between July and October carries a 1st installment delinquent date of December 10 and a 2nd installment delinquent date of April 10. Supplemental bills mailed between November and June carry a delinquent date based on the month in which the bill was mailed-the 1st installment is delinquent the last day of the month after the bill was mailed. The 2nd installment is delinquent four (4) months later. For example, if a supplemental tax bill is mailed in February, the 1st installment delinquent date would be March 31 and the 2nd installment delinquent date would be July 31.

If I don't pay on time, will I be charged a penalty?

Yes. If your payment is not received or postmarked by the delinquent dates, the penalty amounts are the same as secured taxes:
1st installment penalty = 10% of the 1st installment amount
2nd installment penalty = 10% of the 2nd installment amount plus $40.00

When should I mail my payment to avoid penalties?

When you mail your payment, your payment envelope must possess a United States Postal Service postmark on or before the tax delinquent date in order to avoid the delinquent penalties.

Is a private postage meter date the same as the United States Postal Service postmark?

No. California law requires the Tax Collector to accept the US postmark, not a private meter date, as the date of payment.

How are the supplemental tax amounts determined?

The difference between the new value and the old January 1 value is multiplied by a proration factor. The proration factor is the percentage of months remaining in the fiscal year. This result is then multiplied by the tax rate (usually 1% plus voter approved indebtedness) to determine the supplemental tax amount due.

Why did I receive more than one supplemental tax bill?

You may receive more than one supplemental tax bill, depending on when you purchased your property or completed new construction. Because the secured property taxes are based on the January 1 value and cover a fiscal year, your purchase/construction date may effect the calculation of the taxes for two fiscal years. For example, if you purchased property in February 2001, your purchase would effect the 2000-01 fiscal year taxes (February 2001 thru June 2001) and the 2001-02 fiscal year taxes (July 2001 thru June 2002). The latter fiscal year is effected because the value would have already been established by the Assessor on January 1, 2001, prior to the purchase/construction. Therefore, the taxes calculated for 2001-02 would also have to be adjusted by a supplemental bill to reflect the increased value.

When should I expect my supplemental tax bill(s)?

Most supplemental bills are mailed within nine (9) months after a change in ownership or new construction. You should receive a Notification of Supplemental Assessment approximately sixty (60) days before the bill is mailed.

Who should I call if I have questions?

Nature of Question Department Phone
Values Assessor 650-363-4500
Tax Rates Controller 650-363-4777
Tax Payments Tax Collector 650-363-4142
Tax Bills Tax Collector 650-363-4142

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2008 Tax Collector, County of San Mateo
Please email comments to taxmaster@co.sanmateo.ca.us