Sonoma County, CA: Property Tax Appeal Instructions (2025-26)

Key facts at a glance

The 60-second primer

California enrolls the lower of (1) your Prop 13 factored base-year value, or (2) market value on January 1 (the lien date).

If market value on January 1 is lower, you may qualify for a temporary Prop 8 reduction for that year. That temporary value is reviewed annually and can rise more than 2 percent when the market rebounds, but never above your Prop 13 track. For a deeper state overview, see the BOE's Prop 8 and assessment appeals guides.

If you own property in multiple counties, see our county-specific guides for all California counties.

New to property tax appeals? Start with our Appeal 101 guide.

The Sonoma "Informal" route (Prop 8 review)

Sonoma does not publish a countywide Decline-in-Value e-form. You can still ask the Assessor to review your value informally. To understand the difference between these approaches, check out this comparison of informal versus formal appeals. If you want appeal rights for the current year, do not miss the formal filing window.

Step 1: Check what you have to beat

Look up your parcel and confirm the enrolled value you're contesting. Note that the test is market value as of January 1. Use the county's parcel search to verify your current assessment.

Step 2: Build tight comps around the lien date

Aim for 3-5 sales similar in bed/bath, GLA, lot, condition, and location. Sales can be no more than 90 days after the valuation date for evidence at hearing, so anchor close to January 1, per BOE Publication 30. For more details on selecting and analyzing comparable sales, read our guide on finding strong comps.

Step 3: Share your packet

Email or call the Assessor's Real Property team with your summary and support. If you do not receive a satisfactory adjustment, pivot to a formal appeal before the deadline.

The Sonoma "Formal" route (Assessment Appeal)

If the informal ask does not resolve it, file a formal appeal with the Assessment Appeals Board (AAB).

Step 1: Timing and what to appeal

For the 2025-26 year, Sonoma's regular filing period is July 2 to Dec 1, 2025. For Supplemental or Escape assessments, you have 60 days from the notice date.

Step 2: File the application

Download BOE-305-AH and mail or hand-deliver it to the Clerk of the Board. Fee is $30 per application. One APN and one roll year per application. If you are appealing the regular roll for current year, choose Decline in Value (Prop 8) as the basis.

Step 3: Who hears your case

Residential cases may be assigned to a Hearing Officer or scheduled before the three-member AAB. Sonoma County code provides for both the AAB and assessment hearing officers. If you prefer a full Board, do not elect a Hearing Officer.

Step 4: Submit evidence the Board actually uses

Upload or provide a clean packet focused on January 1. Comparable sales should not be later than 90 days after the valuation date, per BOE Publication 30. You may request a pre-hearing Exchange of Information under R&T §1606 to see the Assessor's case. Initiate at least 30 days before the hearing.

Step 5: Hearing day and decisions

You present first, then the Assessor. Decisions can be mailed after the hearing; a stipulated agreement can resolve the case without appearing. Sonoma notes a typical 6-12 month wait from filing to hearing, although law allows up to two years.

DIY Evidence Checklist (short and potent)

Here's what makes a strong packet:

Comparable sales: 3-5 truly comparable sales within approximately 0.5-1 mile, similar size, age, and condition. Contract dates clustered near Jan 1. Avoid sales more than 90 days after the valuation date, per BOE guidance.

Documentation: Photos of condition issues that existed on Jan 1. Written estimates for deferred maintenance.

Analysis: Simple adjustment grid showing how each comp relates to your home. A one-page narrative that explains neighborhood trends and your adjustments.

Optional strategy: Request Exchange of Information to preview the Assessor's packet.

Mini comp grid you can copy

(Keep your set within 90 days after Jan 1 if used at hearing, per BOE Publication 30.)

Frequently asked questions

Does Sonoma have a free online informal review form?

No. The county does not publish a Prop 8 e-form. You can contact the Assessor to discuss a decline-in-value, but to secure appeal rights you must file with the AAB during the formal window.

What if I miss the regular filing window?

You cannot appeal that year's regular roll value. You can still appeal a Supplemental or Escape assessment within 60 days of its notice, or wait for the next regular window.

Can I use spring sales for a January 1 appeal?

Yes, if they are within 90 days after January 1, per BOE Publication 30. Earlier sales are acceptable too. Boards favor sales closest to the valuation date.

Will my taxes jump more than 2 percent after a reduction?

They can. Prop 8 reductions are temporary and reviewed annually. When market value rises, your assessment can increase by more than 2 percent until it returns to your Prop 13 track. We address these concerns in detail in this piece about whether appeals can backfire.

How long until my hearing?

Sonoma reports a 6-12 month backlog, with a statutory maximum of two years to hear cases.

Key Sonoma-specific reminders

2025 deadline nuance: For the 2025-26 year the last day is Dec 1, 2025 because Nov 30 is a Sunday, per the BOE's county filing deadline table. In most years it is July 2 through Nov 30.

Mail, do not email, your application: Sonoma's process is paper filing by mail or hand delivery. Include the $30 fee.

Use the county parcel tools: Verify your assessed value and pull basic roll data before you build comps using the county parcel search.

How AppealArc helps

We turn "my house is over-assessed" into a county-ready packet. AppealArc finds same-neighborhood, same-vintage sales, trims the set to lien-date-friendly comps, and outputs a clean grid plus a one-page narrative that assessors and boards actually read. You focus on signing; we focus on January 1 proof.

Official sources

Rules are local. Always verify the current instructions with Sonoma County and county's official sites before filing.