A Guide to the Supplemental Assessment Process

​​On July 1, 1983, Senate Bill 813 amended the state Revenue and Taxation Code to create what are known as "Supplemental Assessments." This new law changed the manner in which changes in assessed value were billed by requiring that any increase or decrease in taxes due to a change in ownership or completed new construction became effective as of the first day of the month following the date of change in ownership or the date new construction was completed rather than on the next annual tax bill. Supplemental assessments result in tax bills ​​​that are "in addition to" (that is, supplemental to) the annual property tax bill sent to property owners in October. Changes in ownership or completed new construction that trigger supplemental assessments are referred to as "supplemental events."

Further information is available on California State Board ​of Equalization, Supplemental Assessments.​

Supplemental Assessments Brochure​ / Printer Settings Recommended - Flip on short side.

A supplemental event results in a reassessment. A reassessment may be an assessed value increase resulting in a supplemental bill(s), an assessed value decrease resulting in a supplemental refund(s) or retaining the same assessed value (no change). To simplify this guide, we will refer to the supplemental assessment process as though it results in a supplemental bill.

Depending on the date of the supplemental event, either one or two supplemental tax bills will be produced. Supplemental events that occur between January 1 and May 31 will generate two supplemental bills. Supplemental events that occur between June 1 and December 31 will generate one supplemental bill.

You may also receive more than one supplemental tax bill if more than one supplemental event has occurred in a fiscal year. If this occurs the bills are pro-rated between each owner for the period of time the property was owned.

This Supplemental Tax Estimator search tool is maintained by an independent data service. After searching and selecting a parcel number, click on the Supplemental Tax Estimator tab to calculate an estimate of your supplemental tax.

​A supplemental tax bill is generated whenever a property is reassessed due to a change in ownership (a sale, transfer, or transfer of fractional interest); or undergoes new construction. The supplemental tax bill reflects any increase or decrease in property tax generated by the supplemental event.​

Bills are typically generated after the triggering event has been reappraised. There is no statutory timeline for their issuance as there are for annual bills. It may take up to 2 years for our office to appraise new construction and changes in ownership as a result of our existing heavy workload.

If your supplemental assessment results in a net decrease you may receive a supplemental refund. Once the new assessment is enrolled, the Assessor’s Office will mail a Notice of Supplemental Assessment letter on the last Friday of the month. The Assessor’s Office is then required by law to hold the assessment a minimum of 30 days to allow the owner time to request a review of the assessed value or file for any applicable exemption. Once more than 30 days have passed from the Notice date, on the first Tuesday of the first full week of a month, the Assessor’s Office electronically transmits the assessment to the Auditor and authorizes the negative supplemental assessment. This begins the Auditor’s Office processes. The Auditor will not issue a supplemental refund until both installments of the annual bill are paid in full. Once the annual bill is paid and the assessment is transmitted and authorized, it may take an additional 90 business days before the Auditor will issue a supplemental refund.

New constr​uction is:

  • Any improvement to real property, such as adding a room, pool or garage;
  • Any alteration which restores a building or other improvement to the "substantial equivalent of new" (such as completely renovating a building);
  • An alteration that changes the way a property is used (e.g. a residence is converted to a retail store, or a garage is converted to living area);
  • Only the newly constructed portion may be reassessed.

Changes in ownership are:

  • The sale or transfer of a property.
  • It should be noted that certain forms of property transfer are not subject to reassessment. Exceptions include:
  • Interspousal transfers 
  • The addition of joint tenants 
  • The transfer, sale or inheritance of certain properties between parents and their children or grandchildren and an application for exclusion is filed with the assessor
  • Transfers between registered domestic partners

For further information or exclusion claim forms, please contact the Assessor's Office Property Transfer staff at (916) 875-0750.​

The Assessor determines the fair market value of that portion newly constructed or changed ownership based on the event date. Once the new assessed va​​lue of your property has been determined, the Assessor will send you a Notice of Supplemental Assessment that will show the new assessed value as well as the net supplemental assessment amount.


  • New value at date of purchase or completion of new construction: $320,000
  • Prior assessed value on current main tax roll: - 200,000
  • The Net Supplemental Assessment increase will be: + $120,000
  • If the net supplemental difference is a positive amount, a bill will be generated if the bill amount is over $10.
  • If the net supplemental difference is a negative amount, a refund will be generated if the amount of refund is over $10 and the annual tax bill has been paid in full.​

Appeals of supplemental assessments must be filed with the Assessment Appeals Board (not the Assessor) within sixty (60) days of the mailing date shown on the supplemental bill or the supplemental refund check

If you feel a supplemental assessment is incorrect, we recommend that you discuss the assessment with the Assessor as soon as possible after receiving your "Notice of Supplemental Assessment" and prior to filing an appeal. If you can provide the Assessor with convincing evidence that the assessment was incorrect, the assessment could be corrected without an assessment appeal hearing.

Important: Filing an appeal does not relieve the applicant from the obligation to pay the taxes on the subject property on or before the applicable due date shown on the tax bill.

Further information about the appeals process may be obtained by contacting the Appeals Board at (916) 874-7894.

Occasionally a supplemental event before yours is changed in the same assessment year due to an appeal or exclusion filing by the prior owner. This is not an error, however, if the previous assessed value changes, the net difference between the prior owner’s new assessment and your already processed assessment would also change. If a previously issued refund is no longer valid as a result of that change, a bill will be issued to recapture those funds. Example:

  • The prior owner received the property through foreclosure on December 5, 2008, and was assessed at $200,000. A Notice of Supplemental Assessment was mailed to the prior owne​​r at the end of December 2008.
  • You purchased the property on December 30, 2008, and were assessed at your purchase price, $120,000. A Notice of Supplemental Assessment showing a minus $80,000 net difference was mailed to you a month later at the end of January 2009.
  • As allowed by law, the prior owner filed an Application for Changed Assessment with the Appeals Board in early January 2009. The Assessor’s Office won’t receive this appeal information or process the appeal until several months later.
  • The 2008-09 annual bill was paid in full in April 2009.
  • In June 2009, the Auditor’s Office sent you a supplemental refund based on the minus -$80,000 net difference for your 6 month period of ownership in the 2008-2009 tax year (January 1 to June 30, 2009).
  • In September 2009 the prior owner’s appeal was processed. The Assessor agreed to reduce the prior owner’s assessed value to your purchase price, $120,000, since both supplemental events occurred in December 2008.
  • As a result, the net difference between the prior owner’s new assessment (now $120,000) and your existing assessment ($120,000) is now a zero net difference.
  • The supplemental refund you received in June 2009 based on the minus -$80,000 net difference is no longer valid.
  • You will receive a new Notice of Supplemental Assessment showing zero net difference and in approximately 60 to 90 days later you will receive a tax bill to repay the now invalid refund.
  • Your tax obligation for the 2008-2009 is still based on your assessed value and your period of ownership.

Yes. The supplemental tax bill is sent in addition to the annual tax bill and both must be paid.

No. Mortgage servicing agencies do not receive the supplemental tax bill. Supplemental bills are sent directly to the property owner. Only annual tax bills mailed in October are sent to lenders.

If you purchase and then resell property within a short period of time and the Assessor has not already issued a supplemental assessment for the date you first acquired the property, any supplemental tax bills will be prorated between you and the new owner. In that case, you would receive a tax bill that reflects only the actual time period you owned the property. The new owner would receive a separate supplemental tax bill reflecting the period of ownership from the date he or she acquired the property until the end of that fiscal year.

If the supplemental bill for your acquisition of the property is issued before a subsequent sale of the property, it is a valid bill and the county cannot prorate the bill between you and the new owner. Any proration of the supplemental bill you do receive then becomes a private matter to be resolved between buyer and seller.

The formula for calculating a supplemental refund is shown below:

  • ​​(Amount of Net Supplemental Assessment) x (Tax Rate) x (Monthly Factor)

The Amount of Net Supplemental Assessment is the new assessed value minus the prior assessed value minus any exemptions allowed.

The Tax Rate is 1% plus recapture factor for any voter-approved bonded indebtedness. Most tax rates in Sacramento County are between 1% and 1.20%. 

The Monthly Factor represents the number of whole months remaining in the fiscal year after the month of the supplemental event. The chart below shows these factors. 

Month      (Factor)
January     (.42)
February    (.33)
March        (.25)
April          (.17)
May          (.08)
June         (1.00)
July           (.92)
August       (.83)
September  (.75)
October      (.67)
November    (.58)
December    (.50)

Additional information regarding tax rates and the computation of supplemental assessments may be viewed at the Dept. of Finance Tax Collector, Finance Tax Information ResourcesAuditor-Controller Tax Accounting Frequently Asked Questions or California Revenue & Taxation Code section 75.41.​

A. The Supplemental Event Occurred between January 1 and May 31

  • A supplemental assessment is always generated for the fiscal year in which the event occurred.
  • An event occurring between January 1 and May 31 will generate a second supplemental assessment for the subsequent fiscal year affected.
  • The second bill is generated because the annual roll assessment created for the coming fiscal year does not reflect the change in value generated by that event, but must also to be adjusted to reflect the difference as well.

B. Prior Owner Had a Supplemental Event in the Same Fiscal Year

  • You can also receive multiple supplemental bills in situations where a series of supplemental events take place within the same fiscal year for different owners. If the bill for the prior owner’s supplemental event is for the same fiscal year in which you took ownership, you will receive a pro-rated portion of that bill for the time period you owned it.

C. Multiple Supplemental Events Occurred While You Owned the Property

  • ​If multiple supplemental events (changes in ownership and new construction) occurred while you own the property, you will receive one or two supplemental bills for each of these events. Supplemental assessments are generated for each assessment.

Bill payment dates are printed on the tax bills. Penalties are added if payments are not made by these deadlines.

The date a supplemental tax bill becomes delinquent depends upon when the tax bill is mailed by the Tax Collector. If the bill is mailed between July 1 and October 31, it becomes delinquent on December 10 for the first installment, and on April 10 for the second installment (the same schedule as for the annual tax bill).

No. The full amount of each installment must be paid to the Tax Collector's Office. Partial payments will not be accepted.

You may be eligible to receive the Homeowners’ Exemption of up to $7000 of assessed value on a supplemental tax bill if the property you acquired was not already receiving the exemption on its annual roll bill and the property you acquired will be your principal place of residence.

A Homeowners’ Exemption is not granted automatically. You must submit a claim form to the Assessor for the Homeowners’ Exemption no later that the 30th day following the date of notice printed on Assessor’s Notice of Supplemental Assessment to receive a full exemption, or an exemption of 80% of the full amount will be granted if you file a claim form by the first installment due date.

To be eligible for the exemption you must occupy the home as your principal residence within 90 days of the purchase date or completed new construction for a newly built residence.

The Homeowners’ Exemption can only be applied to a supplemental assessment that is a net increase and results in a supplemental tax bill. It cannot be applied to a supplemental assessment lowering the value.

Supplemental taxes are eligible for the same property tax exemptions and assistance programs as are annual tax bills. In addition to the Homeowners’ Exemption, you may apply to the Assessor's Office for other exemptions such as the Disabled Veterans, church, and welfare that may result in greater tax savings if you qualify. You must apply to the Assessor for these exemptions no later than the 30th day following the date of notice printed on the Assessor’s Notice of Supplemental Assessment. For further information regarding these other exemptions, contact the Assessor's Office Institutional Exemptions staff at (916) 875-0720​.

You may contact our Customer Service staff at (916) 875-0700, visit our office at 3636 American River Dr, Suite 200 Sacramento, CA 95864-5952, or view our Topics A to Z General Information index.