Supplemental Notices and Tax Bills

This third-party tool can be used to estimate the value of your supplemental tax bill based on a change of ownership. It is an estimate only, and the actual assessment values and resultant tax bills will be determined by the Sonoma County Assessor’s Office and the Auditor-Controller-Treasurer-Tax Collector in compliance with all state laws.

Note: The Supplemental Tax Estimator services are provided by a third party and is not maintained or warranted by the County of Sonoma. No liability is assumed for the accuracy of the data provided. The County of Sonoma assumes no responsibility arising from use of this information. All Data is Provided Without Warranty of Any Kind, either expressed or implied, including but not limited to, the implied warranties of merchantability and fitness for a particular purpose.

Supplemental Tax Estimator

Supplemental Assessments may be issued when a property changes ownership or new construction is completed. If you received a Notice of Supplemental Assessment, the Assessor was required to reassess your property.

This Supplemental Assessment will reflect the difference between the prior assessed value and the new assessed value (the value used to calculate your tax bill). It covers the time from the ownership change or completion of new construction to the following June 30th, and in some cases the subsequent fiscal year. It is important to understand that these Supplemental Assessments are ‘in addition’ (supplemental) to the annual property tax bill. Your regular tax bill is still due and payable at its stated time

In some cases, you may receive two or more supplemental tax bills. The number of supplemental bills is dependent on the date the ownership changed, or new construction was completed. If the date is:

June 1 to December 31: You will receive 1 supplemental assessment prorated from your date of event to the following June 30th.

January 1 to May 31: You will receive 2 supplemental assessments. The first will be prorated from your date of event to the following June 30th. The second will be for the entire subsequent assessment year from July 1 to June 30th.

The Assessor determines the fair market value of the portion of property that changed ownership or was newly constructed based on the event date.

Once the new assessed value of your property has been determined, the Assessor will send you a Notice of Supplemental Assessment that will show the prior assessed value, the new assessed value, and the difference between those two values which is the net supplemental assessment amount.

More about how Property Values Are Assessed

Appeals of supplemental assessments must be filed with the Assessment Appeals Board (not the Assessor) within sixty (60) days of the date on the notice or the postmark date for the notice, whichever is later.

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. Filing deadlines are firm, so please contact the Clerk of the Assessment Appeals Board regarding filing requirements and deadlines.

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.

More about Assessment Appeals

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

More about Supplemental Taxes

There are three reasons why an owner may receive more than one supplemental tax bill:

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.

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.

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.

Read more about Supplemental Taxes