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About The Assessor's Office

Deva Marie Proto Clerk-Recorder-Assessor

I am honored and excited to be serving as your Clerk-Recorder-Assessor and Registrar of Voters. Our office provides so many core vital services to the public and we believe in the importance of doing so fairly, efficiently and with great customer service.

I look forward to building on the successes we have had in the past, while embracing many future improvements.

Deva Marie Proto 

What Does the Assessor Do?

There are approximately 183,000 parcels of land in Sonoma County. The Assessor's Office is responsible for determining the assessed value of each parcel. California's property tax laws require each property to have a value for property tax purposes. We currently have a staff of 64 who perform this complex, challenging and rewarding job.

The Assessor’s Office administers the California property tax laws in a fair way for all property owners. We make public service a top priority. This office emphasizes cooperation with and assistance to the public. We work hard to make the Sonoma County Assessor's Office the "exception to the public's idea of government service."

Property Taxes, Bills and Payments

The Tax Collector' Office creates and mails property tax bills and collects property taxes and is not part of the Assessor's Office. Property tax laws are established by the California Constitution. The California State Board of Equalization have established property appraisal rules and standards used by California Assessors.

Appraisal Support

The Appraisal Support Unit monitors, records and processes property values, building permit activity, property descriptions, and assessment appeal applications. They help property owners with these subjects, as well as supplemental assessments and roll corrections.

Appraisal Support staff update and maintain records in the Assessor's digital property record system. For example, they compute property values as determined by the appraisal staff, and then update the property system with the resulting values. They also maintain a computerized building permit inventory for the appraisal staff, track assessment appeal applications, and input and maintain property description records.


The Mapping Unit is responsible for maintaining assessment parcel maps. There are nearly 7,000 maps showing ownership and tax rate area boundaries for the purpose of assessment. Map Drafting Technicians use computer aided drafting (CAD) software to create, revise and update parcel maps. They use information gathered from recorded deeds, subdivision maps, parcel maps and records of survey.


The Ownership Unit examines approximately 35,000 recorded documents annually. They determine whether transfers of title are subject to reappraisal according to the Proposition 13 statutes. Staff enter all transfers of title into the computerized property information system to ensure correct owner information for property tax purposes.

This division processes claims for exclusions such as the Parent/Child Transfer Exclusion and the Residential Base Year Value Transfer Exclusion. 

Customer Service & Exemptions

The Customer Service and Exemptions Unit is responsible for helping taxpayers by phone and at the front counter. They maintain the counter map books and coordinate the sale of parcel maps, Use Code Booklets, and other non-confidential property information.  The staff also update and maintain property and mailing address information in the Assessor's property system.

Customer Service staff monitors, records and processes claims for exemptions like:

This division also provides support services to the Standards, Mapping and Appraisal divisions and maintain approximately 180,000 property records.

Real Property Appraisal

The Real Property Appraisal Unit is responsible for appraising over 30,000 parcels of real property for assessment purposes every year. They appraise residential, income, agricultural, commercial and industrial properties. Staff answer questions about property appraisal and help the public various issues. Here are some of the duties staff perform.

  • Appraising water and other utility companies
  • Appraising possessory interests
  • Appraising agricultural preserves
  • Appraising mineral rights
  • Appraising geothermal properties
  • Appraising taxable public properties
  • Testifying at assessment appeals hearings
  • Business and Personal Property

Read more about how How Property Values are Assessed

Business Property Appraisal

Many types of equipment used in business is subject to property tax in California. Boats, vessels and airplanes are classified a personal property and is taxed. The Business and Personal Property Appraisal Division values all types of business property from that owned by small family businesses to that owned by large firms and agricultural operations. All privately owned boats and airplanes in Sonoma County are also valued.

The valuation of a business is based on the annual Business or Agricultural Property Statement filed by the taxpayer. This division also audits the accounting records of selected businesses to verify the accuracy of the data reported on the property statements.

The Assessor’s Office values and maintains assessment information for approximately 20,000 businesses, 15,000 boats, and 900 private aircraft. 

Read more about Business Property, Boats and Aircraft Assessments