California Supplemental Assessment Guide (2026)

If you recently bought a home in California or built an addition, you have probably received a confusing extra tax bill called a supplemental assessment. It arrives months after closing, often catches new homeowners off guard, and is one of the most appealed items in California property tax.

This guide explains what a supplemental assessment is, how it is calculated, when you owe it, and how to appeal it if it looks wrong.

What Is a California Supplemental Assessment?

A supplemental assessment is a one-time tax bill that captures the difference between your property's Prop 13 base year value before and after a qualifying event — usually a change of ownership or new construction. It is authorized by California Revenue & Taxation Code §75.

When you buy a home or finish new construction, your property gets reassessed at current market value as of the event date. The regular annual property tax bill won't pick up that new value until the next fiscal year (which starts July 1). The supplemental assessment fills the gap — it bills you for the difference between the old assessment and the new one, prorated for the partial year you owned the property at the new value.

When You Will Get a Supplemental Assessment

• You bought a home — the most common trigger; you owe a supplemental on the difference between your purchase price and the previous owner's base year value
• You completed new construction — additions, ADUs, swimming pools, major remodels that add value (RTC §75.5)
• You received property through a transfer that triggers a new base year value (most non-spousal transfers post-Prop 19)
• You changed ownership structure — placing property into certain trusts, partnerships, or LLCs may trigger a supplemental if the change is reportable under §62

You will NOT get a supplemental for:

• Transfers between spouses or registered domestic partners
• Transfers into a revocable living trust where you remain the beneficial owner
• Routine annual inflation adjustments (those go on your regular bill)
• Cosmetic improvements like painting, flooring, or roof replacement that don't add taxable value

How a Supplemental Assessment Is Calculated

The supplemental assessment is the difference between:

• The new base year value (your purchase price or completed cost of construction), and
• The prior assessed value (the previous owner's factored base year value)

That difference is then prorated for the number of months you owned the property between the event date and the end of the fiscal year.

Example: You buy a home on October 15, 2026 for $1,100,000. The previous owner's factored base year value was $480,000.

• Supplemental assessment: $1,100,000 − $480,000 = $620,000 of newly assessed value
• Tax rate (Alameda County): approximately 1.18%
• Full-year supplemental tax: $620,000 × 0.0118 = $7,316
• Prorated for the months remaining in the fiscal year (Nov–June = 8 months of 12): $7,316 × 8/12 = $4,877

That $4,877 supplemental tax bill is on top of the regular property tax bill, which is also recalculated for the next fiscal year at the new base year value.

When the Supplemental Bill Arrives

Supplemental bills typically arrive 4 to 12 months after the triggering event. The exact timing depends on:

• How quickly the change of ownership is recorded with the County Recorder
• How quickly the Assessor's office processes the change
• Your county's billing schedule (some counties batch supplementals quarterly)

You may also receive two supplemental bills if your purchase straddles fiscal years. For example, a closing in May 2026 generates one supplemental for the May–June portion of FY 2025–26, and another for the full FY 2026–27 if the regular bill for that year was already calculated at the old value.

The first installment of a supplemental is due on the same day as your regular property tax — usually November 1 (delinquent after December 10) or February 1 (delinquent after April 10). Read your bill carefully; the due dates depend on when the Assessor mails it.

How to Appeal a California Supplemental Assessment

You have 60 days from the date the supplemental notice is mailed to file an appeal (RTC §1605). This is shorter than the regular annual appeal window of July 2 – September 15.

Grounds for appealing a supplemental:

• Sale price is higher than fair market value — for example, you paid above-market in a bidding war, or the recorded sale price includes seller credits/concessions
• Construction cost is overstated — Assessors sometimes use building permit values that exceed the actual cost of completed work
• Wrong property characteristics — Assessor's records show wrong square footage, bedroom count, or lot size
• Qualifying transfer exclusion was missed — Assessor didn't apply spouse, trust, or parent-child exclusion that you qualify for

You file the appeal using Form BOE-305-AH (Application for Changed Assessment) with the Clerk of your county's Assessment Appeals Board. The filing fee in most counties is $30–$60 per parcel.

Why Most Supplemental Appeals Succeed

Supplemental assessments are more appealable than regular annual assessments because:

• They are based on a single transaction price, which is sometimes inflated by seller concessions or non-arms-length factors
• Assessors don't always have time to verify property characteristics for every transaction
• The "value as of date of event" standard creates room for argument on properties that may have already been declining at the time of sale
• Construction cost data is often outdated by 6-12 months when the Assessor uses it

According to California State Board of Equalization data, well-documented supplemental appeals have success rates in the 60-80% range when accompanied by comparable sales evidence.

Strategic Considerations

• Don't ignore the bill while you appeal. Pay the supplemental on time and request a refund if you win. Failure to pay can result in delinquency penalties even if your appeal is ultimately successful.
• The 60-day clock starts at mailing, not receipt. Open every envelope from the Assessor immediately. Missed deadlines are not extendable.
• Sale prices above $50,000 over the median for the area are a strong appeal candidate — they often reflect bidding-war premiums that don't represent true fair market value.
• If you bought during a market peak that has since declined, your supplemental and ongoing Prop 13 base year value may already be above current market — both are appealable.

How TaxDrop Helps

TaxDrop handles California supplemental assessment appeals as part of our standard property tax service. We review your purchase documents, identify any inflated sale-price or construction-cost figures, and file BOE-305-AH with your county's Assessment Appeals Board within the 60-day deadline. We charge 25% of first-year tax savings — no upfront cost and no fee unless we save you money.

Get your free California supplemental assessment review →