What sets your property taxes, and what doesn't.

A bigger number in the letter feels like a bigger bill coming. It usually isn't.

Every year or two a letter arrives from the town assessor with a new number on it: what the town now says your home is worth. When that number goes up, most people read it as a warning. A home worth more on paper feels like a bigger tax bill on the way.

It does not work that way. Under Proposition 2½, the property-tax law Massachusetts voters passed in 1980, a rising assessment mostly changes the tax rate, not what you owe.[1] The same law explains a fact that sounds impossible: the wealthiest towns in the state, the ones that send the largest tax bills, tax their property at the lowest rates in it.

Two numbers everyone watches, the rate on the bill and the value in the letter, turn out to carry almost no information. Here is what does.

The law caps the town's total, not the rate and not your house.

Two words do all the work here: the levy and the rate. The levy is the total amount of money a town raises from all of its property taxes put together, one pot for the whole town. The rate is the price of the tax, the dollars owed per thousand dollars of assessed value. Your bill is your assessment times the rate, and nothing else.

Proposition 2½ caps the levy. Not the rate, and not the value of any single house. A town's levy can rise only 2.5 percent a year, plus a little extra for brand-new construction, unless the voters agree to lift the cap.[1]

Hold that cap in mind and watch what a reassessment does. Every year or two the assessor updates what each property is worth to track the market. When values across town jump, the levy cannot follow, so the rate drops to fit.

The rate is not a decision about how hard to tax. It is whatever number makes the capped levy come out of this year's values. It moves so the levy does not have to.

A town-wide reassessment is close to a wash.

If every home in town rises the same 20 percent, every rate falls about 20 percent, and almost nobody's bill changes. The town is dividing the same pie. A revaluation just remeasures the slices.

Your own bill moves for one reason the assessment letter never mentions: whether your home rose faster or slower than the town as a whole. Outrun the town average and your slice of the pie grows, so your bill rises. Lag it and your bill falls. This is the one real catch in the whole system, and it is a small one, because it is zero-sum. For every homeowner whose bill goes up because their block got hot, another's goes down. The town still collects the same total.

The tool lets you feel it. Put in your assessment and your current bill, then move the two sliders. Set them equal and push the town-wide number as high as you like. Your assessment can double while your bill barely stirs.[2] Only when your own home outruns the town does the bill move, and then only by the gap between the two.

The wealthiest towns tax their property at the lowest rates.

The rate does the same disappearing act from one town to the next, and it produces a result that ought to be impossible if the myth were true. Line up the 345 Massachusetts cities and towns with clean numbers and compare two things for each: how much property it has per resident, and the rate at which it taxes that property. The relationship runs backward. The more valuable a town's property, the lower its tax rate.[3]

The reason is arithmetic, not virtue. A rate is the levy divided by the town's total property value, so two towns can collect identical dollars per resident and the one with pricier homes will post the lower rate, because it is dividing by a bigger number. And that number keeps swelling. Between FY2022 and FY2026, the assessed value of the typical Massachusetts town rose about 41 percent while the capped levy could rise only about 10.[4]Each revaluation, the rate dropped to keep the levy inside the larger value. It was not tracking anyone's decision about taxes. It was chasing the housing market.

Watch it at the edges. In Marblehead, where property runs about $468,000 a resident, the effective tax rate is 0.90 percent. In Beverly next door, at about $255,000 a resident, it is 1.23 percent.[5]Marblehead's property is worth roughly 1.8 times Beverly's per person, and it taxes that property at about three-quarters of Beverly's rate. Go to the ends of the state and the pattern only sharpens. The lowest rates in Massachusetts belong to its most expensive addresses: Chilmark, on Martha's Vineyard, taxes at about 0.21 percent, Nantucket at about 0.34 percent. The highest rates belong to some of its poorest cities: Springfield near 2.13 percent, North Adams near 2.25 percent.[6] The poorest places tax hardest and the richest tax softest. Read as a measure of how much a town asks of its property, the rate gets the story exactly backward.

The rate has one honest job. A second, separate limit in the law says a town's levy can never exceed 2.5 percent of the full value of everything in it, a ceiling rather than a yearly step.[7] A town whose rate climbs toward that ceiling is near a wall, which is why the highest rates in the state all sit just under 2.5 percent and stop. Only a handful of communities are anywhere close to that ceiling, and no town on the North Shore is. For nearly everyone, the rate is a number to ignore.

So why is almost everyone sure it runs the other way? Because the wealthy town does send the bigger bill, and that half is true: a modest rate on an expensive house is still a large pile of dollars. People see those bills, and the big houses behind them, and reach the obvious verdict. The myth is really two true facts, that wealthy towns pay large bills and that assessments keep climbing, wired together by a mechanism that is not there. Pull it out and the two sit harmlessly side by side. Rich towns pay large bills because their houses are worth a lot, not because their rate or their appetite is high. Assessments rise because the market rises, and the rate quietly falls to meet them.

A rising assessment is not a sign your neighbors are doing well.

There is one more thing the number in the letter is not. It is not a sign that the people in your town are doing well. Property values and residents' incomes drift apart. Track how much each has grown across Massachusetts over the last decade, with inflation stripped out, and the two barely relate: a town's values can climb while its households' earnings stall, or the reverse.[8]

In Swampscott, on the North Shore, property values rose about 22 percent in real terms over that span while the income of the typical household did not move at all.[9] The houses got more expensive. The people living in them did not get richer. In other towns the opposite happened, incomes rising while values slipped.

So a rising assessment can mean your neighborhood is prospering. It can also mean buyers from somewhere else are bidding up the housing while the families already there are priced further out. The number in the letter cannot tell you which. It measures the house, not the household.

This is the same law working underneath. The revenue a town collects rides a formula set in 1980, and it changes only through a few narrow channels, slowly. What swings freely, independently and out of sight, is the value of the property and the income of the people. A town can look transformed on paper while collecting almost exactly what its formula always said it would.

Three levers decide what a town collects. The rate is not one of them.

So if it is not the rate and not your rising assessment, what decides how much a Massachusetts town collects? Three things, and close to only three.

Every town is some blend of these three: the hand it was dealt in 1980, how much it has built since, and how willing its voters are to tax themselves past the limit. That blend is a kind of fiscal fingerprint, and it explains far more about a town's finances than any rate ever will. Beverly is one sharp version of it. It has leaned on the second lever and never once pulled the third, growing its tax base steadily while never, in the entire life of the law, passing an operating override or a debt exclusion.[13]

The letter tells you what your house is worth. It was never what set your taxes.

So when the assessor's letter lands and the number has jumped, it helps to remember what the number is. It is an estimate of what your house would sell for. It is not your tax bill, and on its own it barely predicts it. It is not a measure of what your town collects, and it is not proof that your town grew richer. If the whole town rose with you, your bill is going almost nowhere. If your rate fell, that is the law working as designed, not a favor and not a mistake.

The rate on the bill and the value in the letter are the two numbers residents are trained to watch. Both are mostly noise. The signal is underneath, in three levers most people never see: the hand a town was dealt in 1980, what it has built since, and whether its voters have ever gone to the polls to tax themselves more.