From 2021 through 2025, 11,834 commercial-type real estate transfer declarations were recorded in Kane, Lake, DuPage, Will, McHenry and Kendall counties. They are public, filed on the state's PTAX-203 form, and published by the Illinois Department of Revenue.
Run Censum's cleaning rules on that public file and 7,187 remain: 60.7%. The rest are multi-parcel deals, sales the filer flagged as non-market, low-dollar transfers, or one deal recorded as several filings at the same price.
That is before anyone checks building size, the field the state form does not carry. This piece walks through the four traps and the rules Censum's collar commercial work uses.
Quick Answer
State sale filings are the best free source of collar county commercial sales, but about four in ten commercial-type filings are not usable as comparables as recorded. Drop filings with the form's own non-market flags, filings covering more than one parcel, sales under $100,000, and same-price filings on different parcels within a week. Then get building size from county records, because the form has none, and drop anything under $10 or over $1,500 per square foot. Compare what is left with the county's implied value, which is about three times assessed value.
What the filing contains
The PTAX-203 is filed with the deed. The public data on data.illinois.gov covers filings since January 1, 2013, updated weekly. Each record carries:
- Line 2 and 3: the number of parcels and each parcel number, with lot size or acreage. There is no building area.
- Line 7: whether the property was advertised for sale.
- Line 8: current and intended use. Codes d through i cover apartments, office, retail, commercial and industrial buildings.
- Line 10: circumstances of the sale, from related parties and partial interests to court-ordered, foreclosure, short, bank-owned and auction sales, trades, sale-leasebacks and "other."
- Lines 11 to 13: full price, personal property included, and net price for the real estate.
For non-residential sales over $1 million, buyers also file the PTAX-203-A. It asks how long the property was marketed, whether it was occupied, whether the buyer was already a tenant, whether seller financing affected the price, and whether the net price was a fair reflection of market value.
Trap 1: one deal, several filings, one price
A portfolio sale is often recorded as a separate filing for each property, each showing the whole portfolio price. Read alone, each filing looks like a single sale at a price far above that building's value.
- 277 filings shared an exact price and recording date with another filing in the same county, in 130 groups of up to five.
- 725 filings shared a price with a filing on a different parcel within seven days.
Censum drops every sale in that second group. The rule is blunt on purpose. Of the 725, only 173 shared a buyer or seller name with the matching filing, because portfolio buyers often use a different company for each property. And 317 were at round multiples of $100,000, where unrelated sales can match by chance. Losing a few real sales costs less than treating one building's share of a portfolio as its market price.
Trap 2: multi-parcel sales
2,710 filings (22.9%) listed more than one parcel, and 325 involved part of a parcel. The price covers everything, so it cannot be tied to one building.
Censum drops multi-parcel filings, and also drops a single-parcel sale if the same parcel appeared in a multi-parcel filing within 90 days. That catches a parcel that changed hands in a package deal around the same time.
Trap 3: non-market transfers
1,269 filings (10.7%) carried a Line 10 flag that points away from an open-market price. The most common were "other" (475), a financial institution or government agency as buyer or seller (232), related parties (214) and partial interests (182).
Two more signals matter after the flags:
- Not advertised. 2,453 of the 7,187 cleaned sales (34.1%) said the property was not advertised. Censum keeps them as arm's length, but treats them as weaker evidence at a hearing.
- The PTAX-203-A answers. Of 5,029 filings with the supplement, 125 said the price was not a fair reflection of market value, 737 said the buildings were empty at sale, and 370 said the buyer already occupied the property. A vacant building or a tenant buying its own space may not compare to an occupied, marketed sale.
Trap 4: size that doesn't match the building
The form gives lot size, not floor area, so price per square foot needs a building size from somewhere else. Censum uses the county's posted building area where it exists (Lake, and Kendall's county records) and the federal National Structure Inventory estimate elsewhere. It drops any record without a real size of at least 300 square feet, or without a location.
Even then, a size can be a building's footprint rather than its floor area, which understates a multi-story building. Censum's last screen catches the worst cases: it drops sales under $10 or over $1,500 per square foot. Under $10 usually means a partial interest or a land deal. Over $1,500 usually means a data error.
In Censum's 2015 to 2026 collar set, that screen took 7,423 clean single-property sales with size and location down to 7,407. Typical prices per square foot ran from $76 for industrial to $182 for retail.
From sale price to the assessment
Outside Cook, property is assessed at one-third of fair cash value. So the county's implied market value is about three times the assessed value. All six collar counties set fresh values in 2027; see the 2027 collar reassessment staffing calendar. The assessment notice itself shows the assessment divided by the district's median level from the state's three-year sales study.
Example: a 20,000-square-foot industrial building assessed at $500,000 implies about $1.5 million, or $75 a square foot. That is the number to compare with cleaned comparables of the same type, size and area.
Comps rank candidates; they do not prove a case on their own. In Censum's test on 965 later sales, a time-adjusted comparable-sales estimate landed within 25% of the actual price 38% of the time. A property's own recent clean sale is stronger evidence. When comparable sales are the proof, PTAB's rules call for at least three, with documents showing their similarity and proximity.
Where Censum fits
Censum Docket's firm-only commercial desk shows collar county commercial leads with comparable state sales and a one-page evidence sheet for firms, built on the cleaning rules above. Censum does not file appeals, is not a law firm and never takes a share of legal fees. The attorney picks the comparables and makes every legal judgment.
The numbers behind this article
Public counts are Censum's tabulation of the Illinois Department of Revenue's PTAX-203, 203-A and 203-B dataset (data.illinois.gov, it54-y4c6), accessed November 11, 2026: filings recorded January 1, 2021 through December 31, 2025, in the six collar counties, with Line 8 current use d through i. Line 10 flags counted: b through k and p through r. Multi-parcel means more than one parcel on Line 2, additional parcel numbers, or a split parcel. The cleaning order was: Line 10 flags, multi-parcel, net price under $100,000, same price on a different parcel in the same county within seven days, then the 90-day multi-parcel check. Buyer and seller names were compared only to count matches; no names are printed. The 7,423, 7,407, price-per-square-foot medians and 38% test figure are aggregates from Censum's internal collar commercial work on its copy of the state sales file, which it keeps private; no individual private records are shown.
FAQ
Is PTAX-203 data public?
Yes. The Department of Revenue publishes filings since 2013 on data.illinois.gov, updated weekly.
Does the PTAX-203 show building square footage?
No. It shows lot size or acreage, and unit counts for apartments. Building area must come from county records or another source.
Why drop same-price sales on different parcels?
A portfolio is often recorded as several filings each showing the total price. Dropping matches within a week removes most of them, at the cost of a few coincidences.
Are unadvertised sales arm's length?
Not necessarily non-market, but weaker. Censum keeps them if no Line 10 flag applies, and marks them.
What multiplier turns a collar assessment into market value?
About three, because the statute sets assessments at one-third of fair cash value.
Does Censum file or represent clients?
No. Censum LLC is independent property tax data and software. The attorney files and makes every legal judgment.
Sources
- Illinois Department of Revenue PTAX-203, 203-A and 203-B data, data.illinois.gov
- Form PTAX-203, Illinois Real Estate Transfer Declaration (R-10/24)
- Form PTAX-203-A, Supplemental Form A
- 35 ILCS 200/9-145, level of assessment
- 35 ILCS 200/1-55, one-third of fair cash value
- 35 ILCS 200/12-30, mailed assessment notices
- 86 Ill. Adm. Code 1910.65, PTAB documentary evidence
- Kane County Board of Review 2026 Rules and Procedures
- 2026 Rules of the Lake County Board of Review
- Related county guides: Kane, Lake, DuPage, Will, McHenry and Kendall
Sources accessed November 11, 2026.
Next step
Put your next collar commercial comparable grid through the four checks, and see how Docket's commercial desk builds the evidence sheet. See Censum Docket
Censum LLC is a data and analytics company, not a law firm and not a tax advisor. Censum is independent and is not affiliated with Cook County or any government agency.