Ada County, Idaho
Shrink-swell clay limits houses without basements on 20% of Ada County's mapped land, a higher share than 70% of US counties and the 7th highest of 43 Idaho counties. A shallow water table limits basements on 4%.
68.2% of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 55 flood losses here since 2016, 36% of them in buildings with basements. 28% of homes were built before 1980, against 50% nationally; the median home dates from 1997.
Data as of October 13, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, October 13, 2026
of homes built before 1980
Soil under Ada County
| Expansive clay (limits slab homes) | 19.9% |
| High water table (limits basements) | 4.3% |
| Septic fields very limited | 87.6% |
| Clay swell (LEP, by area) | 4.5% |
| High-swell land (LEP 6%+) | 20.1% |
| Urban land, not rated | 2.5% |
| Mapped soil | 678,810 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Chilcott | 10% | n/a |
| Purdam | 7% | n/a |
| Power | 5% | n/a |
| Scism | 4% | n/a |
| Tadpole | 4% | n/a |
| Elijah | 4% | n/a |
How we count · survey areas ID001 (2025-08-27)
What the numbers mean here
Clay swell averages a linear extensibility of 4.5% by land area here, and 20% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 20% of the mapped land.
Clay shrinks as it dries: 68.2% of the county is in drought on the latest map, and 20% of Ada County's mapped land has shrink-swell clay.
Rain over the 12 months to August 2026: 11.64 in, 0.41 in below the 1991-2020 average year of 12.05 in.
USDA rates 88% of the land very limited for septic absorption fields.
Drought
68.2% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 22 of the last 52 weeks; the most was 68.4% of its area, in 5 weeks, most recently on the July 14, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 68.2% |
| D2 severe or worse | 14.7% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
55 flood losses since 2016, $306,510 paid. The worst year was 2017, with 37. Foundations of the buildings, where recorded: 36% basements, 20% slabs, 36% crawl spaces, 8% elevated.
| Year | Flood losses |
|---|---|
| 2016 | 3 |
| 2017 | 37 |
| 2018 | 0 |
| 2019 | 2 |
| 2020 | 2 |
| 2021 | 2 |
| 2022 | 2 |
| 2023 | 2 |
| 2024 | 2 |
| 2025 | 1 |
| 2026 to date | 2 |
Rain
| Average year, 1991-2020 | 12.05 in |
| 12 months to August 2026 | 11.64 in (-0.41) |
| 2025 | 13.50 in (+1.45) |
| Wettest year since 1895 | 1983: 19.85 in |
| Driest year since 1895 | 1966: 6.87 in |
| Days with 1 inch or more, normal at BOISE AIR TERMINAL (8 mi away) | 0 |
Homes
| Housing units | 211,440 |
| Median year built | 1997 |
| Built before 1980 | 28% |
| Built 2000 or later | 45% |
| Owner-occupied | 71% |
| Median value, owner-occupied | $512,300 |
| Single-family detached | 74% |
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 |
|---|---|---|---|
| Canyon County, ID | 26 mi | 1% | 14% |
| Elmore County, ID | 39 mi | 7% | 2% |
| Gem County, ID | 43 mi | 19% | 9% |
| Boise County, ID | 45 mi | 3% | 2% |
| Payette County, ID | 46 mi | 4% | 9% |
| Owyhee County, ID | 61 mi | 27% | 2% |
| Malheur County, OR | 71 mi | 19% | 4% |
| Camas County, ID | 74 mi | 19% | 22% |
Sources: USDA SSURGO (August 27, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status