California Supplemental Property Tax Bill: How It Works

A California supplemental property tax bill is an extra, one-time bill that catches your taxes up to your purchase price or newly finished construction. It is not a duplicate of your annual bill, and in most cases your mortgage escrow will not pay it. When you buy a home, the assessor resets its value to market value as of the purchase date. The annual bill was based on the seller’s older, usually lower value, so the county bills the difference for the months you own the home. If you closed between January 1 and May 31, expect two of these bills. Here is how it is calculated, when it is due, and who pays it.

The short answer

• It bills the gap between the new value and the old roll value. The tax is figured on the difference between your new base year value and the value already on the roll, not on the full purchase price.
• It is prorated by month. State law sets a proration factor based on the first day of the month after you closed or construction was completed. For example, a March closing uses a factor of 0.25 for the current fiscal year.
• A January–May event produces two bills. One covers the rest of the current fiscal year. The second covers the whole next fiscal year, because that year’s roll was set using the January 1 value, before your purchase.
• You pay it, not your lender, unless you arrange otherwise. The bill is mailed to the owner. Lenders do not get a copy and usually do not pay it from escrow.
• The deadlines are short. You have 30 days from the supplemental notice to claim the $7,000 homeowners’ exemption against it, and generally 60 days from the mailing date to appeal the value.

What triggers a supplemental assessment

California’s Prop 13 system taxes a property based on its value when it last changed hands or was built, plus at most 2% a year for inflation. When a property changes ownership, or when new construction is completed, the assessor must appraise it at full cash value as of that date. That value becomes the new base year value under Revenue and Taxation Code §75.10 (opens in a new tab). See our Prop 13 and Prop 19 guide.

The annual roll is set once a year as of the January 1 lien date. The supplemental roll makes up the difference for the time between the event and the next annual roll that reflects it. The Board of Equalization’s supplemental assessment page (opens in a new tab) says the prorated bill “covers the period from the first day of the month following the supplemental event to the end of the fiscal year.” California’s fiscal year runs from July 1 to June 30.

The assessor and the tax collector are separate offices. The assessor first mails a Notice of Supplemental Assessment. That notice shows the new base year value, the old roll value, the event date, the supplemental amount, and the date it was mailed (R&T §75.31 (opens in a new tab)). The tax bill comes later from the tax collector. Los Angeles County’s secured property tax FAQ (opens in a new tab) says processing takes about six months.

Why you may get one bill or two

R&T §75.11 (opens in a new tab) sets the rule by date:

• Event between June 1 and December 31: one supplemental assessment. It is the difference between the new base year value and the value on the current roll. The next annual roll, set on January 1, will already reflect your value.
• Event between January 1 and May 31: two supplemental assessments. The first covers the rest of the current fiscal year. The second covers the entire next fiscal year. That year’s roll was set on January 1 using the seller’s value, so the second bill is the difference between your new value and that roll value.

Several events in the same year can add more bills. If two bills look alike, compare the fiscal year and event date printed on each before you assume one is a mistake.

Proration factors by month

R&T §75.41 (opens in a new tab) fixes the factors. The event is presumed to have happened on the first day of the month after the actual date. A closing on March 3 and a closing on March 28 are both treated as April 1. The county applies a full year’s tax rate to the supplemental assessment, then multiplies by the factor.

June is a special case. Because the presumed date is July 1, the statute says no supplemental assessment is made on the current roll. Instead the full factor of 1.00 applies to the new fiscal year. If the computed supplemental tax is $20 or less, the auditor may cancel it.

Worked example: a March purchase

Hypothetical example. The tax rate is an assumption, not a real rate for any county. You close on a house on March 12, 2026, for $850,000. The assessor sets the new base year value at $850,000. The seller’s value on the 2025–26 roll was $420,000. We assume the 2026–27 roll, set on January 1, 2026, carries the seller’s value forward with a 2% inflation adjustment, to $428,400. We assume a total tax rate of 1.1%: the 1% Prop 13 base rate plus 0.1% for local voter-approved debt. Your bill shows your county’s actual rate.

Together the two supplemental bills come to $5,820.10. That is on top of your share of the regular annual bills. For 2026–27 you would also get a regular annual bill on the $428,400 roll value, about $4,712.40 at the same assumed rate. The annual bill plus supplemental bill 2 equals $9,350, which is 1.1% of $850,000. That is the full tax on your purchase price for the year. From 2027–28 on, your annual bill starts from the $850,000 base year value, and the supplemental bills stop.

The seller’s regular taxes for the months you owned the home in 2025–26 are usually split between buyer and seller on the closing statement. That proration covers the annual bill only. It does not cover the supplemental bills, which did not exist yet at closing.

To estimate your own bill, try Santa Clara County’s supplemental tax estimator (opens in a new tab). Its disclaimer (opens in a new tab) lists situations the tool can’t handle, such as unusual ownership and exemption cases.

When supplemental bills are due

Supplemental taxes are due on the date the bill is mailed. They are paid in two installments. Under R&T §75.52 (opens in a new tab), the delinquency dates depend on the month the bill is mailed, not on your closing date:

For November through June mailings, the first installment is delinquent at the end of the month after mailing. The second is delinquent at the end of the fourth calendar month after that. A late installment gets a 10% penalty, and a cost charge is added once the second installment is late. If a date falls on a weekend or holiday, the deadline moves to the next business day. The statute also guarantees at least 30 days to pay without penalty. If both bills in the example were mailed in September 2026, they would be due by December 10, 2026, and April 10, 2027.

Supplemental taxes are a lien on the property from the date of the purchase or completion (R&T §75.54 (opens in a new tab)). If they stay unpaid past the second installment, the property can go into tax default (§75.53 (opens in a new tab)).

Why escrow and your lender usually don’t pay it

The supplemental bill goes to the owner, not the lender. The Board of Equalization says “lending agencies do not receive the original or a copy of the supplemental tax bill even if they are otherwise being sent and are paying the owner’s annual tax bills.” The Los Angeles County Treasurer and Tax Collector (opens in a new tab) says lenders “do not usually pay” supplemental bills. If you have an impound account, contact your lender as soon as the bill arrives.

Your servicer may agree to pay it if you send a copy. Ask for a written answer that names the bill number, the amount, and the date they will pay. If they agree, watch for an escrow shortage later, since your monthly escrow payment was not set up for it. Our escrow shortage guide explains how that catch-up works. If they won’t pay, pay the collector directly before the delinquency date. If a servicer says it paid but the county shows the bill unpaid, follow the steps in our guide to a servicer that did not pay property taxes.

Negative supplementals and refunds

A supplemental assessment can be negative. This happens when the new base year value is lower than the value on the roll. For example, a home might sell for less than its factored Prop 13 value after prices fall. In that case the auditor refunds part of the taxes paid on the annual roll instead of sending a bill. The assessor’s notice must say so (§75.31(e) (opens in a new tab)).

Under R&T §75.43 (opens in a new tab), the auditor must make the refund within 90 days after the negative assessment is enrolled on the supplemental roll. If the refund is late, interest is owed. The BOE cautions that a negative supplemental “will not reduce (nor can it be used as a credit toward)” an annual bill that is still due. Keep paying the annual installments on time.

Claim the $7,000 homeowners’ exemption right away

California’s homeowners’ exemption removes $7,000 of assessed value from an owner-occupied principal residence (R&T §218 (opens in a new tab)). At a 1.1% rate that is about $77 a year. The claim form is BOE-266, Claim for Homeowners’ Property Tax Exemption (opens in a new tab), which you file with your county assessor. The seller’s exemption does not carry over to you. A new owner must file a new claim.

• Against the supplemental bill: file within 30 days after the date of the Notice of Supplemental Assessment to get the full exemption (R&T §75.21(c) (opens in a new tab)). The exemption only applies to the extent the property isn’t already receiving it on that roll. For a January–May purchase, one claim covers both supplemental assessments. If you miss the 30 days but file before the first installment of the supplemental bill is delinquent, 80% of the exemption’s tax value is canceled or refunded.
• On the regular annual roll: file by 5 p.m. on February 15 for the full exemption on the fiscal year that begins that July (R&T §255 (opens in a new tab)). If you file after February 15 but by December 10, you get 80% of the exemption, up to $5,600 (R&T §275 (opens in a new tab)).
• Once granted: the claim stays in effect until the property changes hands or stops being your principal residence. Tell the assessor if you move out.

Sonoma County (opens in a new tab), for example, tells owners to apply “within 30 days of a supplemental assessment notice.” Don’t wait for the bill. The 30 days run from the assessor’s notice, which comes first.

New construction supplementals

New construction works the same way, but only the new work is assessed. If you add a room, an ADU, or a pool, the assessor values the completed addition at market value and adds it to your existing Prop 13 value. The supplemental bill covers just that added value. For a new house on land you already own, the supplemental assessment is the value of the new house; the land keeps its existing value.

The trigger is the completion date. Under R&T §75.12 (opens in a new tab), construction counts as complete on the earliest date it is available for the owner’s use. A builder who does not intend to occupy the property can instead use the date it is first occupied or used, but must notify the assessor before, or within 30 days after, construction starts. Use as a model home does not count. The same month-based factors and the two-bill rule for January–May completions apply. Our guide to a new construction home’s first property tax bill covers the land-only bills that often come before the supplemental.

How to appeal the supplemental value

What you appeal is the new base year value behind the bill. For a recent arm’s-length purchase, the price you paid is often strong evidence of market value. Appeals are most useful when the assessor used the wrong price, included personal property or non-real-estate items, used the wrong event date, or valued new construction above what it adds to the home.

• Most counties: file the assessment appeal application within 60 days of the mailing date printed on the Notice of Supplemental Assessment, or its postmark, whichever is later (R&T §1605(b)(1) (opens in a new tab)).
• Los Angeles County and counties that have opted in: the 60 days run from the mailing date printed on the supplemental tax bill, or its postmark, whichever is later (§1605(c) (opens in a new tab)). The Los Angeles County Assessment Appeals Board (opens in a new tab) describes its deadline as 60 days from the mailing date on “the Supplemental notice or tax bill,” whichever is later.
• If the notice never reached you: if you did not receive the notice at least 15 days before the deadline, you may file within 60 days of the tax bill’s mailing date with a signed declaration that the notice was not received in time (§75.31(c)(3) (opens in a new tab)).

You can also ask the assessor for an informal review, but that does not extend the appeal deadline. Filing an appeal does not delay the bill either. Pay on time, and you will get a refund if the value is reduced. The BOE assessment appeals FAQ (opens in a new tab) covers the general process.

What to do after you close

1. Note your closing date and find its factor in the table above. If you closed between January and May, plan for two bills.
2. Estimate the amount with your county’s estimator or the formula: (new value − roll value) × tax rate × factor. Set the money aside now.
3. File BOE-266 as soon as the sale records. Don’t wait for a notice. If a notice arrives, file within 30 days of its date.
4. Check the assessor’s notice for the value, event date, and mailing date. Put the 60-day appeal deadline on your calendar if anything looks wrong.
5. Make sure the county has your mailing address, especially if you are not living in the home yet. Penalties can be canceled only in limited cases, such as when the county failed to mail the bill to the address on the roll.
6. Send each bill to your lender, or pay it yourself, before the delinquency date printed on it. Keep the receipt or the collector’s posted payment record.

Use our California county directory or the county finder to reach your assessor and tax collector. The new homeowner property tax checklist covers the rest of your first year.

Frequently asked questions

Why did I get two supplemental tax bills?

Your purchase or construction was most likely completed between January 1 and May 31. The first bill covers the rest of that fiscal year, through June 30. The second covers the whole next fiscal year. That year’s annual roll was set on January 1, before your purchase, so it still shows the old value. Both bills are valid, and neither is a duplicate.

Does my lender pay the supplemental tax bill?

Usually not. Lenders do not receive a copy, and most impound accounts only cover the annual bill. Send the bill to your servicer and ask in writing whether it will pay. If not, pay the county yourself by the delinquency date.

Can I get a refund from a supplemental assessment?

Yes, if your new value is lower than the roll value. The auditor issues the refund, generally within 90 days after the negative assessment is enrolled. It is not credited against an annual bill you still owe.

How long do I have to appeal a supplemental assessment?

Generally 60 days from the mailing date on the assessor’s notice, or its postmark if later. In Los Angeles County and counties that have adopted the alternative rule, the 60 days run from the tax bill’s mailing date. Check your notice for your county’s rule.