Caldwell, ID Accessory Structures: ADU Rules (2026)

Caldwell regulates accessory dwelling units (ADUs) in City Code 10-12-03-1, 'Accessory Buildings, Structures, Uses, and Dwellings.' In April 2025 the City Council amended the code so that adding an ADU is a right for any single-family home property rather than a use that required a special use permit in only certain zones. An ADU must be a minimum of 200 square feet and is limited to one bedroom. Recreational vehicles are prohibited for use as an ADU, mobile homes are prohibited, and manufactured or other modular construction is prohibited unless placed on a permanent foundation that complies with all building-code requirements for a permanent dwelling. The ADU shall not be subdivided from the main parcel on which it sits. For setbacks, the ADU must meet the setback requirements for accessory structures, except when attached to the principal residence, in which case the underlying zoning-district setbacks apply; height follows the height, setback, and area schedule of Chapter 10. ADUs are also permitted in the Steunenberg Residential Historic District, where reporting from the 2025 amendments indicates they are limited to no larger than 75 percent of the original home and must match its design. Idaho's SB 1354 (2025) further requires Treasure Valley cities over 10,000 people, including Caldwell, to update ADU codes by February 1, 2027, eliminating local owner-occupancy mandates.