The tax bill you're underwriting isn't the tax bill you'll pay.
Cook County assessed values consistently move toward the sale price on a triennial reassessment cycle. See how a Cook County rental that pencils on the seller's current tax bill can stop penciling once the assessor catches up — and how PencilOut models that shift before you buy.
Same Chicago 3-flat, two different tax assumptions.
Both scenarios below use identical price, rent, financing, and expense assumptions — the only difference is which tax bill the underwrite trusts. Both are computed live by PencilOut's real, deterministic scoring engine, not hand-typed figures.
The seller's bill was $4,100/year. PencilOut's model projects a post-purchase bill of $9,030/year — a difference of $4,930/year that shows up as roughly $411/month in lost cash flow. Underwriting on the seller's bill alone turned a real risk into an invisible one.
Built on Cook County's own assessment and sale-price data.
PencilOut ingests Cook County Assessor and Treasurer records directly — assessed values, parcel sale history, and tax bills going back years, covering Cook County parcels (Chicago and suburbs) — rather than relying only on whatever tax figure a listing or the seller reports. That data spine is what makes the reassessment model above possible.
Two methods produce the modeled post-purchase tax figure, in preference order:
- Calibrated (preferred). An empirically-derived re-basing coefficient, specific to the property's Cook County class code and township, computed from real historical assessed-value/sale-price pairs in that cohort. Used only where enough comparable sales exist to trust the number. The Chicago (City-triad) coefficients have been validated against a leave-one-cycle-out backtest; suburban Cook (North- and South-triad) coefficients, once available, carry an explicit "newly fitted, not yet backtested" disclosure until an equivalent suburban backtest exists.
- Flat rate (fallback). A market-average effective tax rate applied to the purchase price — the same method used everywhere the calibrated tier doesn't have enough data yet, and for larger multifamily property (5–49 units), which Cook County values by income capitalization rather than sale-price rebasing.
Every analysis shows which method produced its number — nothing is presented as more certain than the underlying data supports, and a separate confidence rating always accompanies the score.
An internal difference-in-differences analysis — comparing 182,123 sold Cook County parcels against matched, comparable unsold parcels in the same class code and township across the 2015–2024 triennials — found that roughly one-fifth of the observed assessed-value movement is attributable to the sale itself, rising to roughly two-fifths for the most under-assessed properties (the ones an investor is most likely to be evaluating). The rest is the same triennial catch-up that comparable unsold properties see too. PencilOut's calibrated coefficient is fit on that combined, observed movement — not just the sale-specific share of it — because that combined figure is what actually shows up on your bill.
Reassessment, answered directly.
What is Cook County property tax reassessment?
Cook County splits its townships into three roughly-equal groups — called triads — each reassessed on its own three-year (triennial) cycle, staggered so that about a third of the county's parcels get reassessed every year. Chicago is the City triad; the North and South triads cover suburban Cook on their own offset schedules. Assessed values for recently-sold properties consistently move toward the sale price by their triad's next reassessment — though most of that movement is part of the same area-wide catch-up that comparable unsold properties see too, not a mechanism that singles out your specific transaction. Either way, the assessed value converging toward the sale price is the reliable, empirical pattern PencilOut's model is built on.
Why does a property's tax bill tend to rise after it's bought?
A property's assessed value often lags well behind its market value, especially after a long hold or a below-market prior sale — and Cook County's regular triennial cycle closes a meaningful share of that gap for every comparable property in the area, sold or not, on its own. A sale adds a smaller effect on top: it gives the assessor a fresh, verified data point — the price you actually paid — which can accelerate how much of the gap closes at the next reassessment. Either way the tax bill tends to rise; PencilOut's model is calibrated on the combined, observed result, not just the sale-specific piece.
When is the next Cook County reassessment?
It depends on which of Cook County's three triads the property's township falls in. Chicago's City triad runs 2024, 2027, 2030, and continues on a three-year cadence after that — the next City-triad reassessment lands in 2027, with mailed and certified values typically rolling out through the following year. The North and South suburban triads are offset from the City schedule and from each other, each on the Cook County Assessor's own published three-year cadence.
How much can my tax bill go up after buying?
It depends on how under-assessed the property was relative to your purchase price, and on the property's class code and township (different areas and property types re-base differently). PencilOut models a per-property estimate rather than a flat citywide rule — see the worked example above for one realistic scenario.
Does PencilOut guarantee the reassessed tax amount?
No. PencilOut's reassessment model is an informational estimate, not a guarantee of assessed value or tax liability, and not tax or legal advice. The figures shown are modeled from real Cook County Assessor and Treasurer data, but only the assessor's office sets the actual reassessed value.
What's the difference between the “flat rate” and “calibrated” reassessment models?
The flat-rate model applies a market-average effective tax rate to the purchase price — a reasonable citywide estimate. The calibrated model instead uses an empirically-derived re-basing coefficient specific to the property's own class code and township, computed from Cook County's own historical assessed-value and sale-price data, when enough sales exist in that cohort to trust the number. PencilOut uses the calibrated model wherever that per-cohort data clears its minimum sample size, and falls back to the flat-rate model everywhere else — it never fabricates a per-property number it can't back with real data.
Does this apply outside Cook County?
No — not yet, and not in the same way even once it does. Cook County is the only Illinois county where a sale factors into reassessment at all; assessed values re-base partway toward the sale price at the property's next triennial. DuPage, Will, Lake, Kane, and McHenry counties run a different system: a quadrennial (four-year) general assessment, with the next one in 2027, and a sale is not a reassessment trigger there under Illinois law — an owner who never sells and a buyer who just closed are treated identically until that county-wide cycle comes around. PencilOut's post-sale reassessment model is Cook-County-specific today; it does not run, and does not claim to run, for those five counties.
See what your next Cook County deal actually pencils to.
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