The Tax Bill Is Frozen. The Market Isn’t.

A few months ago, homeowners in Delaware opened property tax notices that looked less like routine municipal paperwork and more like a margin call.

For decades, Delaware had been taxing homes using old values. New Castle County’s last reassessment dated back to 1983. Sussex County’s went back to 1974. The market moved. Neighborhoods changed. Homes appreciated. But the tax roll stayed frozen in time.

Then the courts forced the state to mark the books.

In New Castle County, average residential assessments rose by roughly 477%. Non-residential assessments rose by much less. The tax burden shifted toward homeowners. Some bills doubled.

It would be easy to describe that as a tax shock. But the shock was not the reassessment. The shock was the 40-year fiction that came before it.

For decades, the paper record said one thing. The market said another. The court simply forced the government to reconcile the two.

That is what property tax reassessment really is: not administrative housekeeping, but a forced mark-to-market of real estate’s most ignored carrying cost.

The quiet fiction inside the tax bill

Most people assume a property tax assessment is a current estimate of value. In many places, it is not close.

Pennsylvania is the clearest example.

In a base-year assessment system, a county picks a year, values every property as of that year, and freezes those values. Counties, municipalities, and school districts can still change millage rates, but those rates apply across the jurisdiction. The individual property value does not automatically update just because the market changes.

This is not California’s Proposition 13, where assessment growth is capped and resets on sale. In Pennsylvania, a new buyer generally inherits the old assessment. A home may have sold yesterday, but its taxable value may still be anchored to an old market.

That sounds taxpayer-friendly. Sometimes it is. But only if your property appreciated faster than the county average.

Neighborhoods do not move in lockstep. A frozen tax base preserves the distribution of value from the year the county stopped looking. Over time, properties that appreciated least become over-taxed relative to market value. Properties that appreciated most become under-taxed. In many places, lower-value homes in weaker markets carry a heavier effective tax burden than higher-value homes in stronger markets.

That is the heart of the lawsuits now working through Pennsylvania.

Pennsylvania is the tell

Pennsylvania is now the state to watch because it is the last major holdout for indefinite base-year assessments.

Roughly 50 of its 67 counties are using assessment years at least 15 years old. Franklin County’s base year traces to 1961. Lackawanna County’s was 1968 before its recent reassessment.

The state publishes one number that reveals the drift: the Common Level Ratio, or CLR. The CLR is the median ratio of assessed value to market value across valid sales in a county. If a county’s CLR is 100%, assessments are roughly aligned with current market values. If it is 50%, properties are assessed at about half of market value. If it is 10%, the tax roll is not just stale. It is an archaeological artifact.

Pennsylvania’s 2025 CLR table is remarkable. Bucks County is at 5.6%. Butler is at 6.0%. Franklin is at 7.2%. Westmoreland is at 8.7%. Pike is at 8.9%. At the other end, counties that just reassessed reset to 100%.

So within one state, some counties are marking property close to market, while others are taxing homes at a nickel or dime on the dollar. That spread tells you where the paper record has drifted farthest from economic reality, where lawsuits are likely to keep coming, and where the next tax bill may already be hiding.

The mark-to-market tax

In capital markets, we understand the difference between book value and market value. If you hold an asset on the books at an old price, and the market moves, the mark eventually matters. The exposure exists whether or not the statement reflects it yet.

Real estate has its own version of that problem.

Call it the mark-to-market tax.

A property’s current tax bill is based on assessed value and millage. But its future tax exposure depends on how that assessed value compares to true market value and to the county’s common level ratio.

If a property is assessed below the county norm, it may be carrying a hidden liability. If it is assessed above the county norm, it may be carrying a hidden asset, including a possible appeal opportunity today.

Take a recently sold house in Point Breeze, one of Pittsburgh’s stronger residential neighborhoods. Round numbers: it sold for about $335,000. Its assessed value is roughly $104,000. That means it is assessed at about 31% of market value. Allegheny County’s current CLR is 49.3%.

So this house is under-assessed relative to the county’s own median relationship between assessment and market value. Its current tax bill is about $2,700, or roughly 0.8% of market value. If the parcel were marked closer to the county norm, the annual tax bill would move toward roughly $4,300.

That is a $1,600 annual gap. For a homeowner, that is household cash flow. For an investor, it is NOI. Capitalized at a 6% cap rate, that hidden tax burden represents something like $27,000 of value.

Nobody can tell you exactly when the mark lands. But you can underwrite it anyway. The risk exists before the reassessment notice arrives.

The mark cuts both ways

Now look at the other side of the ledger.

In McKeesport, another Allegheny County property recently sold for roughly $30,000. Its assessed value is about $26,000. That means it is assessed at close to 88% of market value. Again, the county CLR is 49.3%.

This house is over-assessed relative to the county norm. Its current tax bill is about $1,000 a year, or roughly 3.4% of market value. If it were marked closer to the county relationship between assessment and value, the bill would fall toward roughly $570.

Put the two houses next to each other and the inversion is hard to unsee. The Point Breeze owner is paying roughly 0.8% of market value. The McKeesport owner is paying roughly 3.4%.

That is about a four-times difference in effective tax burden, produced not by a special assessment, not by a local surcharge, not by anything the owner did, but by a frozen snapshot from 2012.

The cheaper house subsidizes the pricier one. That is the system working as designed. And that is exactly what the lawsuits are attacking.

Sometimes the stale mark is an asset

I had the same reaction many owners will have when they first understand this system.

I own a few rentals on the Northampton side of Bethlehem, Pennsylvania. When I first learned how far some counties’ tax rolls had drifted from market reality, my instinct was defensive. Maybe I had a big tax bill waiting for me. Maybe the frozen assessment was quietly flattering my returns.

It turned out to be the opposite.

My property appears to be modestly over-assessed relative to Northampton County’s common level ratio. In other words, the stale mark may be an asset, not a liability. I am now mulling an appeal.

That matters because I had been viewing the property as a marginal hold. The town has not allowed me to add the units to the parcel that I first envisioned when I underwrote the deal, and I had even considered selling. But the prospect of an assessment appeal changes the keep-versus-sell math, at least at the margin.

Any recovered tax drops straight to NOI. Not once. Every year.

A stale assessment is not automatically good or bad. It is a mispriced position. Sometimes the mark is coming for you. Sometimes it is sitting there waiting to be claimed.

Why almost nobody sees it

The data needed to see this problem is public. The parcel record exists. The sale exists. The assessment exists. The tax amount exists. The county ratio exists. But the truth is not sitting in one neat field.

In Delaware, after reassessment hit, Spotlight Delaware wanted to show residents how the new values landed by neighborhood. To build a usable map, it had to file public-records requests across three counties, work with a data lab, and spend roughly $10,000. The public record was technically public. But it wasn't usable.

Even a clean parcel record can mislead you. A field labeled “market value” may simply echo the old assessed value in a base-year county. An “assessment year” may tell you the current roll year, not the actual base year. A tax amount may capture only one layer of the bill, while the school district carries most of the burden.

How to see the mark before it lands

None of this requires a black-box “reassessment risk” score.

Most of what you need is already sitting in a property-detail response. Pull the parcel’s total assessed value, current tax, and an independent estimate of market value. Then compute the ratio that matters: assessed value divided by market value. That ratio tells you how far the book has drifted from reality.

But the parcel record alone will not tell you everything. The assessment year usually means the current roll year, not the year the value was actually set. To understand how stale the basis may be, you need the county’s base year — an administrative fact published by the county assessor, and one we compiled for Pennsylvania alongside this piece.

Then comes the number that turns “this looks under-assessed” into “here is the dollar gap”: the county’s common level ratio. In Pennsylvania, the state publishes that ratio each year. You can also compute a proxy yourself from assessed-value-to-sale-price pairs, if you have the data depth to do it.

The logic is straightforward. If a property is assessed below the county norm, it may be carrying a latent tax increase. If it is assessed above the county norm, it may be carrying a latent tax benefit, including a possible appeal opportunity.

One caution: the tax field is the one people trust too quickly. Tax divided by assessed value gives you an effective rate, which is what the exposure math needs. But do not mistake that derived rate for statutory millage. And do not assume the tax amount always represents the full bill. In some jurisdictions, the clean-looking figure may capture only one levy layer.

Once those pieces are connected, a portfolio’s worth of latent tax exposure becomes something you can rank, filter, and underwrite.

The assessment roll may be the oldest, least-examined dataset in American real estate. That ledger will not stay hidden forever. The gap between assessed value and market value has become too large to ignore. The regressivity is too visible. The litigation template is now established.

For owners, the current tax bill is not enough. For investors, taxes cannot be underwritten as a static expense line. For lenders, debt service coverage may be flattered by an artificially low public burden. For software builders, the parcel record is not the answer. It is the beginning of the question.

The market already knows what the property is worth. The tax bill may not.

The opportunity is in seeing the difference before the government does.