Property tax reassessment: don't underwrite the seller's tax bill (2026)

Assessed value is not market value

Property taxes are charged on a property's assessed value, which is the county's number, not the market's. Assessed value drifts away from what a property is actually worth for two everyday reasons. First, some states cap how fast an existing owner's assessment can climb — California limits the annual increase to 2%, and some other states run their own caps or reassess only every few years. A property held since 2014 can be carried on the rolls at a value that has nothing to do with 2026 prices. Second, owner-occupants often receive a homestead exemption that shaves a fixed amount off the taxable value — a benefit that does not apply to a rental.

So the seller's low tax bill is not a market quirk you get to inherit. It is the product of a capped, possibly years-old assessment plus an exemption you do not qualify for. The bill is real, but it describes the seller's situation, not yours.

What a sale actually triggers

In some states — California and Michigan among them — a change of ownership is a reassessment event. The recorded sale tells the assessor what the property is worth — you just proved it by paying for it — and the assessed value is reset upward toward that price. Exactly how this plays out depends on where you buy, and the mechanics fall into a few patterns worth recognizing:

Acquisition-value states. California's Proposition 13 is the cleanest example: when you buy, the assessor sets a new base-year value equal to your purchase price, then caps growth at 2% a year going forward. The reset can be dramatic because you are replacing a base that may date back decades. Michigan "uncaps" taxable value in the year after a transfer, resetting it to the state equalized value. In both, the sale triggers the reset: right away through a supplemental bill in California, and on the next year's assessment in Michigan.

Cyclical-reassessment states. Some states reassess on a fixed cycle, whose length varies by state, regardless of sales, but a recent sale gives the assessor a fresh, defensible value to apply at the next cycle. The danger here is timing: you may pay the old, low figure for your first year, then watch it leap when the cycle catches up. Underwriting the first-year bill as if it were permanent is a trap.

Either way, the practical rule is the same: unless your county confirms otherwise, underwrite as if a purchase resets your taxes toward your purchase price, and treat any year you pay less as a temporary gift, not the baseline.

The formula — and the shortcut

The full mechanic is three numbers multiplied together, with the mill rate stated per $1,000:

The assessment ratio is the fraction of market value a county taxes (some assess at 100%; Ohio uses 35%; Mississippi uses 10% for owner-occupied homes and 15% for other real property, including rentals), and the mill rate (or millage) is the tax per $1,000 of that taxable base (one mill is $1 per $1,000). You can chase all three down at the assessor's office — and for a precise budget you eventually should — but for underwriting there is a faster move that folds them into one number:

The effective tax rate is taxes actually paid divided by market value — it already bakes in the assessment ratio and the loss of an owner-occupant exemption. You can pull it from the county, or back it out yourself: find a comparable property that sold recently, divide its post-sale tax bill by its sale price, and you have a clean rate to apply to your own deal. Effective rates vary widely from state to state and county to county, so this is a local number, not a national one.

A worked example: the duplex that flips negative

Take a $400,000 duplex. You put 25% down ($100,000) and finance $300,000 at an illustrative 7.5% over 30 years (use your own lender quote). That principal-and-interest payment is roughly $2,098 a month, or about $25,170 a year (check it on the mortgage payment calculator). Each side rents for $1,600, so gross potential rent is $3,200 a month — $38,400 a year — and at a 5% vacancy assumption you collect about $36,480. Outside of taxes, the property costs $7,800 a year to run: insurance, repairs, capital reserves, and water/lawn/admin.

The listing shows property taxes of $3,400 — the seller has owned since 2015 and lives in one unit. You, the investor, will be reassessed toward your $400,000 purchase price; at this market's 1.5% effective rate that is $6,000 a year. One line item, wrong by $2,600. Here is what that single number does to the whole underwrite:

On the seller's bill the deal looks like a thin but real winner: a 6.3% cap rate, a DSCR right at 1.00, and a few dollars of monthly cash flow. Underwrite the tax bill you will actually pay and net operating income drops $2,600, the cap rate sheds nearly two-thirds of a point, DSCR falls to 0.90 — below your lender's minimum (ask what it is) and below the 1.0 line where the property stops covering its own loan — and cash flow swings from +$9 to −$208 a month. Same building, same rent, same price. The only thing that changed was using an honest tax number, and it turned a deal you would sign into one you would walk from.

The cash you forget at closing: the supplemental bill

In reassessment-on-sale states there is a second, smaller surprise. The annual bill resets at the next cycle, but the county also wants the difference between the old and new assessment for the part of the year you already own the place. That comes as one or two supplemental tax bills (California's term; two if you close between January 1 and May 31; check how your state bills a mid-year change), and they land weeks or months after closing — long after most buyers have stopped watching for new costs. On our duplex, a $2,600 annual increase prorated over, say, eight remaining months is roughly $1,700 of first-year cash you did not plan for. It is not a recurring expense, so it does not belong in your operating numbers, but it absolutely belongs in your cash-to-close and first-year reserves.

How to get the real number before you write the offer

You do not need the assessor to bless your figure before you make an offer — you need a defensible estimate, and there are three reliable ways to build one. The fastest is the effective-rate shortcut above: purchase price × the local effective tax rate. The most accurate is to pull a recently sold comparable — a property that changed hands in the last year or two, already reassessed — and divide its current tax bill by its sale price; that rate has the reset already priced in. And the most authoritative is to call the county assessor with the parcel number and ask what the property will be assessed at on a sale at your price, and what the current mill rate is. Do at least the first two on every deal; do the third before you remove contingencies.

A few traps to avoid while you are at it. Do not assume an LLC or a clever title structure dodges the reassessment — a purchase is a change of ownership regardless of who signs. California's proportional transfer exclusion, for example, covers only a change in how the same owners hold title, with each owner's share unchanged — not a purchase from a seller. Do not forget that the exemptions vanish along with the low assessment: a homestead exemption, or any other exemption tied to the owner living there, that the seller held does not transfer to a landlord, and stripping it can raise the bill even before the value resets. And do not treat a low first-year bill in a cyclical state as your run-rate — find out when the next reassessment hits and underwrite to the post-reset number.

This is the same discipline that makes the difference on every soft expense line. Taxes, like insurance, are big, lumpy, and easy to copy wrong from a listing — and both are exactly where an optimistic number hides a bad deal. Run the honest figure through the free TrueCap analyzer and you see the cap rate the property really earns, not the one the seller's tax history flatters.

FAQ

The bottom line

The property-tax line on a listing is the seller's number, not yours, and underwriting to it is one of the most expensive shortcuts in the business. A purchase can reset the assessment toward what you paid and strip the owner-occupant breaks you never qualified for, so the bill you inherit is often higher than the bill you see. Estimate it the right way — purchase price times the local effective rate, cross-checked against a recently sold comp — and budget the supplemental bill as a closing-year cost. The full TrueCap analyzer keeps tax as a manual local input rather than copying the seller's figure, then re-runs cap rate, DSCR, and cash flow on the evidence you enter — so you find out a deal is fragile before you sign, not when the first real tax bill arrives. None of this is tax advice; confirm your specific assessment and exemptions with the county and a local professional before you close.