A supplemental property-tax bill is separate from your regular annual bill. It can follow a qualifying ownership change or completed new construction. California's Board of Equalization explains that the supplemental amount reflects the change in assessed value, with timing rules that determine which fiscal-year period is covered. BOE supplemental-assessment guide.
Correction — September 7, 2026: The previous October example used a partial month. We have replaced it with the BOE's published monthly proration factor and removed unsupported assumptions about bill timing and parcel charges.
A corrected October example
For an event occurring in October, the supplemental tax takes effect November 1. The BOE lists 0.67 as the proration factor for November through June. Its calculation applies the local tax rate to the net supplemental assessment, then the applicable factor. BOE calculation and factor table.
Here is hypothetical arithmetic using that published factor:
| Input | Illustrative value |
|---|---|
| New assessed value | $700,000 |
| Previous assessed value | $400,000 |
| Difference | $300,000 |
| Assumed applicable tax rate | 1.15% |
| Annual tax on the difference | $3,450 |
| BOE factor for an October event | 0.67 |
| Illustrative prorated amount | $2,311.50 |
The assumed rate is for this example only, not a quoted Santa Clarita rate. Obtain the applicable rate, assessed values, exemptions and county calculation for the parcel. Do not automatically add Mello-Roos or other fixed parcel charges to an assessed-value-based percentage.
Why there may be more than one bill
Los Angeles County explains that events between January 1 and May 31 can produce two supplemental bills: one for the remainder of the current fiscal year and another for the following fiscal year. A lower reassessed value can instead result in a refund. LA County supplemental-tax FAQ.
Who receives the bill?
The county mails supplemental bills to the property owner. Its guidance says they generally are not paid from the mortgage impound account and advises checking with the lender. Read each bill's own due dates instead of assuming the regular annual-bill schedule applies. LA County billing guidance.
Documents to request before budgeting
- The current annual tax bill and parcel number.
- The previous assessed value and the relevant ownership or construction date.
- Any county supplemental notice already issued.
- Written clarification from your lender or escrow officer about what their accounts will pay.
Keep those documents together. If a number is unclear, ask the assessor about valuation and the tax collector about billing and payment. A generic percentage of the home's purchase price is not a substitute for that calculation.
Our supplemental-tax budgeting checklist focuses on the documents and follow-up steps. Contact Rose District if you want help organizing questions during a home purchase.
Frequently asked questions
Does an October 15 purchase use half of October?
The BOE's method starts with the following month. An October event has a November 1 effective date and the published 0.67 factor for the remaining period.
Is the example a quote for my property?
No. Its values and rate are hypothetical. Your county notice and parcel-specific information determine your bill.
Will my lender automatically handle it?
Check directly. The county says supplemental bills generally are not paid from an impound account.
Official sources checked September 7, 2026.