Land records · Parcel maps · October 7, 2026
What Is a Supplemental Tax Bill After Buying a Home?
What is a supplemental tax bill? An extra property tax bill issued after a sale or new construction raises assessed value mid-year, common in California.
What is a supplemental tax bill? A supplemental tax bill is an additional property tax bill issued when a property’s assessed value changes during the tax year because of a change in ownership or completed new construction. It is best known in California, where Proposition 13 resets assessed value at sale. The supplemental bill covers the difference between the old and new assessed values for the remaining months of the tax year. New homeowners are often surprised by it because it arrives months after closing and usually is not paid through the mortgage escrow account. This page is general land-records background, not legal advice.
How it is calculated
- New assessed value is set as of the date of sale or completion.
- Old assessed value is subtracted.
- The difference is multiplied by the tax rate.
- The result is prorated for the remaining months of the fiscal year.
If the change happens late in the fiscal year, two supplemental bills may be issued, one for the current year and one for the next.
Why it is often missed
Regular property tax bills are based on the value on a set lien date. When ownership changes mid-year, the regular bill still reflects the prior owner’s value until the next cycle. The supplemental bill catches up. Lenders often do not include it in escrow because it is sent directly to the owner. Owners should check whether their lender will pay it or whether they must.
Negative supplementals
If the new assessed value is lower than the old one, such as after a price decline, the county may issue a refund instead of a bill.
Other states
Some states have similar mechanisms with different names, such as added or omitted assessments for new construction completed during the year. Many states do not issue supplemental bills and simply adjust the next year’s assessment.
Relationship to the PCOR
In California, the preliminary change of ownership report filed with the deed helps the assessor set the new value that drives the supplemental bill. See what is Proposition 13.
Proration at closing
Regular taxes are prorated between buyer and seller at closing based on the current bill. Supplemental bills usually are not, because they reflect the buyer’s new value. Buyers should budget for them.
Reading the bill
The supplemental bill shows the event date, the old and new values, the proration factor, and the amount due. Payment deadlines may differ from regular bills. See how to read a property tax bill.
Example
A buyer purchases a California home in October for $800,000. The seller’s assessed value was $350,000. The difference of $450,000, multiplied by a 1.1 percent rate and prorated for the remaining eight and a half months of the fiscal year, produces a supplemental bill of roughly $3,500 that arrives several months later.
Bottom line
A supplemental tax bill covers the increase in assessed value after a sale or new construction for the rest of the tax year, most commonly in California. It often arrives months after closing and may not be paid by escrow. Budget for it and check with your lender. Find county assessors via the Platbookmapper map.
What is a supplemental tax bill FAQ
What is a supplemental tax bill?
An extra bill for a mid-year increase in assessed value.
Why did I get a supplemental tax bill?
Usually because you bought the home or completed construction.
Does my escrow account pay it?
Often not; check with your lender.
Can a supplemental bill be a refund?
Yes, if the new value is lower.
Do all states issue supplemental bills?
No, it is most common in California.