Cook County, Illinois
A shallow water table limits basements on 59% of Cook County's mapped land, a higher share than 73% of US counties. Shrink-swell clay limits houses without basements on 25%.
None of the county is in drought on the September 22, 2026 Drought Monitor map. FEMA's flood data list 1,057 flood losses here since 2016, 82% of them in buildings with basements. 73% of homes were built before 1980, against 50% nationally; the median home dates from 1961.
Data as of September 22, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, September 22, 2026
of homes built before 1980
Soil under Cook County
| Expansive clay (limits slab homes) | 25.0% |
| High water table (limits basements) | 58.7% |
| Septic fields very limited | 67.0% |
| Clay swell (LEP, by area) | 5.0% |
| High-swell land (LEP 6%+) | 24.6% |
| Urban land, not rated | 29.2% |
| Mapped soil | 616,045 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Ozaukee | 6% | over 99% |
| Markham | 6% | n/a |
| Ashkum | 5% | n/a |
| Milford | 3% | n/a |
| Elliott | 2% | n/a |
| Beecher | 2% | n/a |
How we count · survey areas IL031 (2025-08-31)
What the numbers mean here
Clay swell averages a linear extensibility of 5.0% by land area here, and 25% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 25% of the mapped land.
On 59% of Cook County's mapped land, soil stays saturated within about 6 feet of the surface for part of the year, which is basement depth.
Rain over the 12 months to August 2026: 41.23 in, 2.88 in above the 1991-2020 average year of 38.35 in.
USDA rates 67% of the land very limited for septic absorption fields.
Drought
0% of the county is in drought (D1 or worse) on the September 22, 2026 map. Part of the county was in drought in 15 of the last 52 weeks; the most was 69.7% of its area, in 2 weeks, most recently on the June 9, 2026 map.
| Category, September 22, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 0% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
1,057 flood losses since 2016, $11.1 million paid. The worst year was 2020, with 290. Foundations of the buildings, where recorded: 82% basements, 13% slabs, 3% crawl spaces, 2% elevated. Median water depth outside the building: 6 in.
| Year | Flood losses |
|---|---|
| 2016 | 68 |
| 2017 | 140 |
| 2018 | 86 |
| 2019 | 84 |
| 2020 | 290 |
| 2021 | 67 |
| 2022 | 23 |
| 2023 | 124 |
| 2024 | 41 |
| 2025 | 43 |
| 2026 to date | 91 |
Rain
| Average year, 1991-2020 | 38.35 in |
| 12 months to August 2026 | 41.23 in (+2.88) |
| 2025 | 30.21 in (-8.14) |
| Wettest year since 1895 | 2019: 49.17 in |
| Driest year since 1895 | 1956: 22.76 in |
| Days with 1 inch or more, normal at CHICAGO MIDWAY AP 3SW (13 mi away) | 9.5 |
Homes
| Housing units | 2,276,493 |
| Median year built | 1961 |
| Built before 1980 | 73% |
| Built 2000 or later | 13% |
| Owner-occupied | 58% |
| Median value, owner-occupied | $324,500 |
| Single-family detached | 40% |
What repairs cost
Published ranges from cost guides, not quotes. More sources, and where they disagree.
Nearby counties
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Lake County, IL | 22 mi | 16% | 87% |
| DuPage County, IL | 23 mi | 33% | 90% |
| Lake County, IN | 32 mi | 16% | 77% |
| Will County, IL | 35 mi | 24% | 85% |
| Porter County, IN | 40 mi | 7% | 70% |
| Kane County, IL | 40 mi | 8% | 82% |
| Kendall County, IL | 46 mi | 26% | 85% |
| Berrien County, MI | 47 mi | 0% | 51% |
Sources: USDA SSURGO (August 31, 2025) · US Drought Monitor (September 22, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status