Idaho Code § 40-607
The costs of constructing, reconstructing, maintaining and acquiring rights-of-way for highways in a county highway system and a highway district highway system shall be borne by the responsible highway jurisdiction. This section shall not be construed as preventing counties and highway districts from contracting with the state for engineering or other services provided just compensation is paid. If planning or engineering studies show the existence of a need, a county or highway district may purchase, condemn or otherwise acquire new or additional rights-of-way for a new alignment of or improvement of an existing alignment of an extension of a county or highway district rural major collector highway through cities with populations of less than five thousand (5,000), provided the extension does not eliminate access to adjacent property owners. A county or highway district shall have jurisdiction, with the full authority to construct, maintain and control, over an extension of a rural major collector highway eligible for federal highway funds within a city, when the city population is less than five thousand (5,000). Counties and highway districts may enter into any mutual agreement for the transfer of maintenance and control of the rural major collector highway extension to the city. A county or highway district may contract with an adjoining county or highway district for the construction and/or maintenance of any part of its highway system.
Idaho Code § 40-607