Buying a home can trigger Sacramento supplemental property tax on the increase from the value already taxed on the regular annual bill to your new assessment. The county sends this additional bill directly to you, even if your mortgage payment includes taxes. The regular bill still must be paid. Sacramento County’s billing guidance explains these separate obligations.

Why the seller’s tax bill can understate your cost

Under Proposition 13, a property’s base value generally starts when ownership changes, with subsequent annual inflation adjustments capped at 2%. A longtime owner’s assessment can therefore be far below today’s purchase price. A buyer generally receives a new market-value assessment, so the seller’s tax history can understate the buyer’s ongoing cost. The county’s assessment explanation describes this distinction.

Sacramento County reassesses the property at its value on the acquisition date, using the purchase price when it falls within the fair market range. The regular bill reflects the preceding January 1 assessment; a later purchase does not simply rewrite that bill. The supplemental tax accounts for the difference. The assessor explains how purchases after January 1 affect billing.

The assessor first sends a Notice of Supplemental Assessment showing the new value and assessment difference. Check the purchase date and values when it arrives. The tax collector sends a separate bill requesting payment. See the county’s assessment process.

How the purchase month changes supplemental tax

California’s property-tax fiscal year runs July 1–June 30. Supplemental taxation starts on the first day of the month after the purchase. For an ordinary purchase that increases assessed value, the Board of Equalization’s billing schedule works as follows:

For a January–May purchase, both fiscal years’ regular bills use assessments from before the purchase. The two supplements account for the increase over those values.

The calculation is assessment increase after applicable exemptions × tax rate × monthly proration factor. Proration adjusts the charge for the months covered. California’s Property Tax Rule 255 governs the rate and proration process.

Purchase example: a $650,000 home bought in March

Hypothetical assumptions: A buyer acquires a home on March 15, 2026, for $650,000, and the assessor accepts that price as market value. Assume the value already on both relevant annual rolls is $350,000, with no exemptions, exclusions, other reassessment events or annual value adjustments. Use an illustrative 1.10% value-based tax rate for both fiscal years, including the basic levy and applicable bond debt. This is not a quote for any parcel; direct levies are excluded.

The assessment increase is $300,000. Applying the county’s supplemental-tax formula, the full-year tax increase is $300,000 × 1.10% = $3,300.

These amounts are additional to the regular bills. Under the assumptions, the 2026–27 regular bill’s value-based portion is $350,000 × 1.10% = $3,850. Adding the $3,300 supplement brings that fiscal year’s value-based tax to $7,150—the same as $650,000 × 1.10%.

For cash planning, distinguish the supplemental bills from the ongoing increase. The $4,125 covers 15 months across two fiscal years, rather than the buyer’s first 12 months. The ongoing increase over the seller’s value-based tax works out to $275 a month. Actual roll values, rates and exemptions can change the result.

Why additional supplemental bills can arrive

The purchase-month schedule is one reason for multiple bills. Separate reassessment events can also generate separate bills, such as a purchase followed by completed new construction. A prior owner’s recent purchase or construction can produce a bill prorated between owners if the property changes hands before that bill is issued. If the bill was already issued before resale, proration becomes a private matter between buyer and seller. Sacramento’s process guide explains these cases.

Compare each bill’s event date, fiscal year and assessment difference before treating it as a duplicate.

Will a mortgage impound account pay the supplement?

An impound account, also called a mortgage escrow account, collects money with the monthly mortgage payment for expenses such as taxes and insurance. The amount collected can change when those expenses change. The CFPB explains how these accounts work.

An impound account does not automatically pay Sacramento County supplemental bills. Send the bill to your servicer and confirm who will pay it, from which funds and by what date. A tax proration or credit on the closing statement also does not establish that a later supplement was paid. The county’s tax FAQs address both impounds and closing prorations.

Keep cash available until payment responsibility is clear. Ask whether the monthly tax collection reflects the anticipated new annual assessment and whether any funds already collected are available for the supplement.

Billing can take months. The purchase month determines the period taxed; the mailing date determines the payment deadlines. Follow the dates printed on each supplemental bill. The regular annual bill’s usual December 10 and April 10 penalty deadlines, subject to weekend and holiday extensions, follow a separate schedule. Supplemental billing rules and annual secured-tax rules explain the distinction.

Check the parcel before setting your cash reserve

The example’s value-based calculation does not capture every charge on an annual bill. The value-based rate includes the basic 1% levy plus applicable voter-approved debt. Annual bills can also carry Mello-Roos special taxes, improvement assessments and service charges that require parcel-level review. The Auditor-Controller explains the components.

Direct levies can include landscape and lighting, park maintenance, reclamation districts and delinquent utility charges. Because they are not calculated from assessed value, multiplying the purchase price by a flat rate misses them. Review recurring charges separately from outstanding charges, and ask the levying district about future amounts. The assessor’s direct-levy guide identifies the responsible agencies.

Before setting your reserve, have the lender or escrow officer reconcile three figures: the regular tax payments still due, the estimated supplemental bills and the tax funds already collected. For a parcel-specific estimate or an exemption question, consult the assessor or a qualified California property-tax professional.

Related reading: California housing coverage, housing affordability and planning a home purchase.