Ada County was established on December 22, 1864, with its county seat at Boise. The county was named for Ada Riggs, the daughter of H. C. Riggs, one of the founders of Boise and a member of the Idaho Territorial Legislature. Boise became the capital of Idaho in 1865.
On the east side of the courthouse is a large paved parking lot and a parking garage which service employees, jurors, and the public. Additional employee parking is available in paved parking lots located west of the building. All parking lots and structures on this site are owned and operated by Ada County. Effective October 1, 2010, the public will receive the first hour of parking at no charge and will pay $1.00 for each hour (or portion of each hour) thereafter. Juror parking is paid by the court.