When was Ada County founded?

Ada County was founded in 1864, formed from Boise County, and named after Ada Riggs.

What is the county seat of Ada County?

The county seat of Ada County is Boise.

Is there an online parcel map for Ada County?

Yes. Idaho Department of Water Resources publishes a parcel map of 220,628 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.

Has Ada County's boundary or name changed?

Ada County was created in 1864 from Boise County. Since then the Atlas of Historical County Boundaries records 7 later changes, chiefly involving Idaho County, Boise County and Canyon County.

Is my lot in a flood zone?

We do not have flood maps for Ada County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

County · code 16001

Ada CountyIdaho

Ada County is a county in Idaho with its seat in Boise. It was created on December 22, 1864, from Boise County and named after Ada Riggs.

Founded1864
County seatBoise
Named forAda Riggs
Land area1,052 sq mi + 8.6 sq mi water
Population494,967 2020 Census
Parcels mapped220,628 checked 16 October 2026

Open the free map Click any parcel in Ada County: acreage, parcel id, address and who publishes the map. No sign-in.

Updated 20 October 2026

Census outline · seat marked

History

Formed in 1864 from Boise County

An old deed may name a county that no longer exists, or a boundary that has since moved.

Ada County was created by Idaho Territory from Boise County on December 22, 1864. Over the following decades, it gained territory from Boise and Idaho counties and lost territory to the creation of Washington, Canyon, and Elmore counties. When Idaho became a state in 1890, Ada County continued as a county of the new state.

Boundary changes from the Newberry Library, Atlas of Historical County Boundaries; the 5 that matter most are shown first.

1864
Ada County created Ada created by Idaho Territory from Boise.
from Boise
22 December 1864 · Id. Terr. Laws 1864, 2nd sess., ch. 29/pp. 430-431
1867
Gained territory Ada gained from Boise and Idaho.
from Boise from Idaho
10 January 1867 · Id. Terr. Laws 1866, 4th sess., ch. 46/p. 124
1873
Gained territory Ada gained from Idaho.
from Idaho
10 January 1873 · Id. Terr. Laws 1872, 7th sess., pp. 30-31
1879
Lost territory Ada lost to creation of Washington, and lost to Idaho.
to Washington to Idaho
20 February 1879 · Id. Terr. Laws 1879, 10th sess., pp. 40-42
1890
Became part of Idaho Ada continued as an Idaho county when the state of Idaho admitted to the Union and Idaho Territory eliminated. 3 July 1890 · U.S. Stat., vol. 26, ch. 656 [1890]/pp. 215-219; Van Zandt, 156

Show all 12 changes (7 more, mostly small adjustments)

1864
Ada County created Ada created by Idaho Territory from Boise.
from Boise
22 December 1864 · Id. Terr. Laws 1864, 2nd sess., ch. 29/pp. 430-431
1867
Gained territory Ada gained from Boise and Idaho.
from Boise from Idaho
10 January 1867 · Id. Terr. Laws 1866, 4th sess., ch. 46/p. 124
1873
Gained territory Ada gained from Idaho.
from Idaho
10 January 1873 · Id. Terr. Laws 1872, 7th sess., pp. 30-31
1879
Lost territory Ada lost to creation of Washington, and lost to Idaho.
to Washington to Idaho
20 February 1879 · Id. Terr. Laws 1879, 10th sess., pp. 40-42
1883
Gained territory Ada gained from Boise.
from Boise
31 January 1883 · Id. Terr. Laws 1882, 12th sess., pp. 67-68
1887
Boundary clarified Ada boundaries clarified (no change).
no change
1 June 1887 · Id. Terr. Laws 1887, comp., spec. and loc., 14th sess., p. 22
1890
Became part of Idaho Ada continued as an Idaho county when the state of Idaho admitted to the Union and Idaho Territory eliminated. 3 July 1890 · U.S. Stat., vol. 26, ch. 656 [1890]/pp. 215-219; Van Zandt, 156
1892
Lost territory Ada lost to creation of Canyon. Act approved 7 March 1891, became effective through a vote in the general election of 26 November 1892.
to Canyon
26 November 1892 · Id. Laws 1890, 1st sess., p. 155; Nelson and Webbert, p. 75
1895
Boundary clarified Ada boundary with Elmore redefined (no change).
with Elmoreno change
9 February 1895 · Id. Laws 1895, 3rd sess., pp. 15-16
1899
Boundary clarified Ada boundary with Elmore redefined (no change).
with Elmoreno change
14 February 1899 · Id. Laws 1899, 5th sess., p. 234
1923
Boundary clarified Ada boundary with Elmore redefined to run along range lines rather than from mouth of Bruneau River to mouth of Mores Creek (no change).
with Elmoreno change
13 March 1923 · Id. Laws 1923, 17th sess., ch. 136/pp. 200-202
1948
Lost territory Ada lost to Elmore. Act approved 10 Mar 1947, became effective through a vote in the general election of 2 November 1948.
to Elmore
2 November 1948 · Id. Laws 1947, 29th sess., ch. 150/pp. 352-359; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-103 "Ada Co."/p. 4

Land and parcels

What the maps cover in Ada

Who publishes each layer, how good it is, and when we last checked.

Parcels Good enough

Lot lines and tax-map numbers for 220,628 parcels from Idaho Department of Water Resources. Drawn for tax purposes: usually within a few feet of a survey, but not a survey.
checked 16 October 2026
Address points Good enough

One point per address, 275,279 in all from Ada County.
edited 12 October 2026 · checked 13 October 2026
Buildings Reference

Roof outlines traced from aerial imagery from FEMA Response Geospatial Office with Oak Ridge National Laboratory (ORNL). Good for seeing what is on a lot; not accurate enough to set a fence from.
checked 18 October 2026

Official: from the office that keeps the record
Good enough: a reliable statewide or national copy
Fallback: a guide only; check the official record
Missing: nothing public found yet

People and land

Ada by the Census

Population

494,967 2020 Census count

Housing units

196,901 2020 Census count

Land area

1,052 square miles

Water area

8.6 square miles

People per sq mi

470 population ÷ land area

Source: US Census Bureau, 2020 Census; US Census Bureau, 2020 Gazetteer Files. Not endorsed or certified by the Census Bureau.

Rules and offices

Ada County's official site

We have found the county's site but not its offices yet; start there.

County government

County government
The official website of Ada County.
adaweb.net

See your own lot on these maps

Open Ada County on Parcel Chart's free map and click your parcel. Save it with a free account; the full record of your place (site map, projects, documents) is Pro, in early access.

Open the free map no sign-in Pro early access

Questions

Common questions about Ada County

When was Ada County founded?: Ada County was founded in 1864, formed from Boise County, and named after Ada Riggs.
What is the county seat of Ada County?: The county seat of Ada County is Boise.
Is there an online parcel map for Ada County?: Yes. Idaho Department of Water Resources publishes a parcel map of 220,628 parcels, rated good enough here. You can click any parcel on it in Parcel Chart's free map, no sign-in needed.
Has Ada County's boundary or name changed?: Ada County was created in 1864 from Boise County. Since then the Atlas of Historical County Boundaries records 7 later changes, chiefly involving Idaho County, Boise County and Canyon County.
Is my lot in a flood zone?: We do not have flood maps for Ada County yet. Look up the address on FEMA's Flood Map Service Center, which shows the effective flood map for any lot.

Nearby