Caldwell, ID Accessory Structures: ADU Rules (2026)

Caldwell's zoning code treats accessory dwelling units, or ADUs, as a permitted accessory use under Section 10-12-03-1, the same section governing sheds, garages and other detached structures. An ADU must comply with all adopted international building and fire codes, and in no case can it have a setback of less than 3 feet from any interior side or rear lot line, a lower bar than the 5-foot minimum most larger accessory buildings face. On a corner lot, a detached ADU cannot sit closer to a side lot line adjoining a street than the street-side setback required in the district's dimensional standards table.

Height is capped by the underlying zoning district's height, setback and area schedule rather than a single citywide number: detached accessory structures generally cannot exceed 22 feet in the R-1, R-2 and R-3 districts or 33 feet in the RS-1 and RS-2 districts, and an ADU must fit within whichever limit applies to its district. Like any accessory structure, an ADU cannot be placed in a recorded or unrecorded easement or public right of way, and if it is built at the same time as the primary home, its certificate of occupancy cannot be issued before the principal structure's own certificate of occupancy.

Because Idaho has no statewide ADU mandate overriding local zoning, Caldwell's own setback and height rules, not a state ADU law, control what a homeowner can build.