How accurate are Illinois price ranges?
Every Illinois figure labelled "Price range" on Groundwork passed a sealed test before it was shown: we priced real sales using only data from before each sale, then compared. Figures labelled "Rough estimate" did not reach that standard; their measured miss is listed too. Where a county's test failed outright, its parcels show no figure.
Tested price ranges
Half within: half of the test sales came within this much of the middle of our range (the median miss). Inside the range: the share of scored test sales that fell inside the printed range. A range whose final check is not passed by its due date is withdrawn (the parcel then shows facts only).
| County | Priced | Test | Test sales from | Test sales | Half within | Inside the range | Parcels with this range | Final check |
|---|---|---|---|---|---|---|---|---|
| Cook County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 27,333 | ±11% | 80% | 1,115,131 | Due Apr 2027 |
| Cook County | condominiums | Value test (il-value-1) | Oct 2025–Sep 2026 | 16,327 | ±10% | 80% | 457,241 | Due Apr 2027 |
| DuPage County | homes | Value test (il-value-1) | Oct 2025–Sep 2026 | 10,657 | ±10% | 80% | 297,880 | Due Apr 2027 |
| St. Clair County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 1,506 | ±14% | 81% | 87,572 | Due Apr 2027 |
| Peoria County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 1,437 | ±11% | 78% | 62,046 | Due Apr 2027 |
| Tazewell County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 1,035 | ±11% | 78% | 49,004 | Due Apr 2027 |
| Kendall County | homes | Value test (il-value-1) | Oct 2025–Sep 2026 | 1,633 | ±15% | 81% | 43,523 | Due Apr 2027 |
| Kankakee County | homes | Value test (il-value-1) | Oct 2025–Sep 2026 | 1,035 | ±15% | 79% | 33,422 | Due Apr 2027 |
| Whiteside County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 410 | ±17% | 73% | 21,156 | Due Apr 2027 |
| Jackson County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 212 | ±19% | 78% | 14,452 | Due Apr 2027 |
| Clinton County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 229 | ±14% | 80% | 13,684 | Due Apr 2027 |
| Douglas County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 126 | ±12% | 84% | 7,010 | Due Apr 2027 |
| Ford County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 93 | ±16% | 75% | 5,038 | Due Apr 2027 |
| Kankakee County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 534 | ±14% | 81% | 645 | Due Apr 2027 |
| Macoupin County | homes | Model test (state-avm-1) | Apr–Sep 2026 | 257 | ±19% | 81% | 20 | Due Apr 2027 |
Rough estimates
Where a county's own sales were too few for a sealed test, or its test missed our standard but stayed usable, the card says "Rough estimate" and gives the miss measured where it was measured: in the county, or across the state's pooled sales. Each label says the range covers about 8 in 10 sales.
| Miss measured | Half within | Covers | Parcels |
|---|---|---|---|
| Will County | ±15% | Land and building | 227,120 |
| Kane County | ±15% | Land and building | 148,141 |
| Lake County | ±20% | Land and building | 72,508 |
| Across Illinois | ±20% | Land only | 56,788 |
| Rock Island County | ±15% | Land and building | 50,034 |
| McHenry County | ±15% | Land and building | 38,809 |
| Winnebago County | ±25% | Land and building | 31,342 |
| Madison County | ±25% | Land and building | 29,520 |
| Henry County | ±15% | Land and building | 18,254 |
| McLean County | ±15% | Land and building | 16,616 |
| Christian County | ±20% | Land and building | 10,512 |
| DeKalb County | ±20% | Land and building | 9,782 |
| Iroquois County | ±10% | Land only | 7,042 |
| LaSalle County | ±15% | Land only | 6,501 |
| Grundy County | ±15% | Land and building | 5,764 |
| Williamson County | ±30% | Land and building | 5,707 |
| Wayne County | ±20% | Land only | 5,537 |
| White County | ±20% | Land and building | 5,333 |
| Christian County | ±10% | Land only | 5,332 |
| Boone County | ±15% | Land and building | 4,906 |
| Bond County | ±15% | Land and building | 4,772 |
| Morgan County | ±15% | Land and building | 4,137 |
| Kankakee County | ±20% | Land only | 4,001 |
| Macoupin County | ±20% | Land and building | 3,717 |
| Coles County | ±20% | Land and building | 3,288 |
| Monroe County | ±20% | Land and building | 3,253 |
| McHenry County | ±30% | Land only | 3,083 |
| Vermilion County | ±35% | Land and building | 2,924 |
| Across Illinois | ±40% | Land only | 2,905 |
| Fulton County | ±25% | Land and building | 2,599 |
| Marion County | ±20% | Land and building | 2,567 |
| Iroquois County | ±25% | Land and building | 2,248 |
| Randolph County | ±15% | Land and building | 1,794 |
| Peoria County | ±15% | Land and building | 1,764 |
| Brown County | ±15% | Land and building | 1,656 |
| Cook County | ±20% | Land and building | 1,489 |
| De Witt County | ±25% | Land and building | 1,385 |
| Mercer County | ±20% | Land and building | 1,282 |
| St. Clair County | ±35% | Land only | 719 |
| McLean County | ±10% | Land only | 621 |
| DeKalb County | ±20% | Land only | 440 |
| Tazewell County | ±15% | Land and building | 189 |
| Jackson County | ±25% | Land and building | 124 |
| Whiteside County | ±15% | Land and building | 108 |
| Williamson County | ±40% | Land only | 107 |
| St. Clair County | ±15% | Land and building | 93 |
| Peoria County | ±10% | Land and building | 69 |
| Clinton County | ±15% | Land and building | 28 |
| Douglas County | ±15% | Land and building | 24 |
| Ford County | ±20% | Land and building | 2 |
What did not pass
The model test (state-avm-1, run Oct 17, 2026) checked 104 county and property-type groups on 44,031 sales; 12 passed. The rest show no range from it.
The value test (il-value-1, sealed sales Oct 2025–Sep 2026) checked 229 groups: 4 passed as a tested range, 167 are shown as rough estimates and 58 show no figure.
Buy-for prices: 0 of 6 testable groups passed (il-buyfor-1), so Illinois parcels show no buy-for price.
Sources and dates
Test sales: recorded Illinois sales on file for each county. Test figures: our out-of-time test reports and, where run, sealed final checks (il-value-1, state-avm-1, il-buyfor-1). Parcel counts: our Illinois map on Oct 23, 2026. Estimates are for your own analysis, not an appraisal or a broker price opinion.