You bought a California home, and now you’re trying to understand a supplemental property tax bill. What triggered it? Why does the purchase date matter? And which payment deadline applies?
The starting point is the change in ownership. California Revenue and Taxation Code Section 75.10 requires the assessor to appraise the property at its full cash value on that date. The resulting new base year value is used to determine a supplemental assessment under Section 75.11.
The assessment is placed on the supplemental roll. The tax calculation and payment deadlines follow their own statutory rules.
What triggers a supplemental property tax assessment?
A change in ownership triggers an appraisal that establishes a new base year value under Section 75.10.
For a home purchase, the relevant date is the date the change in ownership occurs. The assessor appraises the property changing ownership at its full cash value on that date. That value becomes the property’s new base year value.
The same section also addresses completed new construction resulting from actual physical construction on the site. In that case, the appraisal date is the date the construction is completed, and the value determined becomes the new base year value of the new construction.
Keep the appraisal and the tax calculation distinct. Section 75.10 establishes the value. The later sections explain how that value produces a supplemental assessment and how the tax is calculated.
If your question concerns a base year value transfer, the Proposition 19 guide addresses that separate topic. The provisions discussed here do not establish the rules for exclusions or exemptions from reassessment.
What is the supplemental assessment based on?
The supplemental assessment is based on the difference between the new base year value and the taxable value on the applicable roll.
Under Section 75.11, the current-roll calculation compares the new base year value with the taxable value on the current roll. When a second assessment is required for a change of the full interest, that calculation compares the new base year value with the taxable value to be enrolled on the roll being prepared.
Those are value comparisons. They are not yet the amount of tax due.
The auditor then uses the supplemental assessment to calculate taxes under Section 75.41. Reading the assessment and the bill with that distinction in mind helps you keep the steps straight: first the value difference, then the tax calculation.
Section 75.11 also provides for a net supplemental assessment when multiple ownership changes occur in the same assessment year. If that applies to your property, ask the county assessor to explain the values used in your assessment.
Why can one purchase produce two assessments?
An ownership change from January 1 through May 31 requires two supplemental assessments under Section 75.11.
The first concerns the current roll. It measures the difference between the new base year value and the taxable value already on that roll. The second concerns the roll being prepared and, for a change of the full interest, uses the taxable value to be enrolled there.
A different rule applies when the ownership change occurs from June 1 through December 31. Section 75.11 calls for one supplemental assessment, based on the difference between the new base year value and the taxable value on the current roll.
The dividing date matters.
A January purchase falls within the two-assessment period. A June purchase falls within the one-assessment period. These are rules about the number of assessments placed on the supplemental roll; they do not establish how a county formats or delivers its documents. Ask the county assessor or tax collector to explain the documents for your property.
How does the purchase date affect the calculation?
The purchase date determines the portion of the tax year reflected in the supplemental tax calculation.
Section 75.41 directs the auditor to apply the current year’s tax rate to the supplemental assessment, first computing the taxes that would be due for a full year. The taxes are computed in two equal installments.
The statute then requires an adjustment using a proration factor to reflect the portion of the tax year remaining. For this calculation, the ownership change is presumed to have occurred on the first day of the following month.
For a purchase that closes in January, the presumed date is February 1. The factor for the current-roll portion is 0.42.
That factor applies to the current-roll portion of the calculation. It does not, by itself, give you the bill amount. For presumed dates from February 1 through July 1, the statute applies a factor of 1.00 to the supplemental assessment for the roll being prepared.
The calculation therefore depends on both the applicable value difference and the timing rules.
When do you have to pay the bill?
Taxes on a supplemental bill are due on the date mailed, with installment delinquency deadlines determined by the mailing month.
Section 75.52 sets out two schedules:
- If the bill is mailed from July through October, the first installment becomes delinquent at 5 p.m. on December 10 of that year. The second becomes delinquent at 5 p.m. on April 10 of the following year.
- If the bill is mailed from November through June, the first installment becomes delinquent at 5 p.m. on the last day of the following month. The second becomes delinquent at 5 p.m. on the last day of the fourth calendar month following the first installment’s delinquency date.
Due and delinquent are different points in this schedule. The bill is due when mailed; the dates above determine when unpaid installments become delinquent.
If a delinquency date falls on a weekend or legal holiday, it moves to the next business day. Read the mailing date carefully, and ask the county tax collector to clarify a deadline you cannot reconcile with the bill.
What happens with small amounts, refunds, or late payments?
The statute allows cancellation of certain small supplemental taxes, provides for refunds on negative assessments, and attaches a penalty to delinquent payments.
Under Section 75.41, the auditor may cancel the supplemental tax if the total is $20 or less. Cancellation is discretionary; the provision does not say the auditor must cancel it.
A negative supplemental assessment follows a different path. The auditor uses the same calculation procedure to work out a refund.
For unpaid taxes, Section 75.52 says a penalty attaches when payment has not been made by the delinquency date and time. It also requires cancellation of the penalty if the assessee or fee owner demonstrates to the tax collector that the delinquency resulted from the collector’s failure to mail or electronically transmit the bill to the address provided on the tax roll.
That is a specific condition for penalty cancellation. It is not a general statement that every missing or unread bill excuses a late payment.
What should you ask about your own purchase?
Ask the county assessor about the assessment values and the county tax collector about the bill, payment instructions, and deadlines.
Useful questions follow directly from the statutory calculation. Which ownership-change date was used? What is the new base year value? Which taxable value was used for comparison? Does the purchase date call for one supplemental assessment or two?
For the bill, confirm the mailing date and both installment delinquency dates. Ask the county about its bill format, where to pay, or how to update a mailing address.
These sections do not establish who pays at closing or whether escrow collects the supplemental tax. They also do not provide a lender’s estimate.
Keep other purchase-related tax questions distinct. The California documentary transfer tax guide covers a different closing tax. For questions about Mello-Roos, see the Mello-Roos guide.
This is general information and not legal advice.
The short version
A California home purchase triggers an appraisal at the ownership-change date, and the resulting value is used in the supplemental assessment calculation.
The purchase date determines whether Section 75.11 calls for one assessment or two. Section 75.41 governs the tax calculation and proration. Once the bill is mailed, Section 75.52 sets the payment and delinquency rules.
Start with the date and the values on your documents. Then check the mailing date and each installment deadline with the county tax collector if anything is unclear.
Curious what your home is worth today? Try the home value tool.
Common questions
What triggers a supplemental property tax assessment?
Under Revenue and Taxation Code Section 75.10, a change in ownership triggers an appraisal at full cash value on the date of the change. That value becomes the new base year value.
Why can a purchase result in two supplemental assessments?
Section 75.11 requires two supplemental assessments for an ownership change from January 1 through May 31. One concerns the current roll, and the other concerns the roll being prepared.
When is a supplemental property tax bill due?
Taxes on the supplemental bill are due on the date mailed. The installment delinquency deadlines depend on the month the bill is mailed.
Can a supplemental assessment result in a refund?
Yes. Section 75.41 directs the auditor to use the same calculation procedure to work out a refund when the supplemental assessment is negative.
