Trinity County, CA Accessory Structures: ADU Rules (2026)

Because Trinity County has no incorporated cities, the county's own rules govern accessory dwelling units everywhere. The county's draft Zoning Code (Public Review Draft, April 2026) Chapter 17.95 carries forward the county's adopted ADU standards to comply with California ADU law, formerly Government Code section 65852.2 and now recodified at Government Code sections 66310-66342. Under Section 17.95.040, ADUs are allowed by right in all zones that allow single-family and multifamily dwellings. A detached ADU may range from a minimum of 150 square feet up to a maximum of 1,200 square feet; an attached ADU may not exceed 50 percent of the existing living area or 1,200 square feet, whichever is less. Setbacks for an ADU are four feet from side and rear lot lines, provided the setback does not preclude an 800-square-foot unit, and height may not exceed 25 feet or the height of the primary dwelling. Section 17.95.030 requires the county to approve or deny a complete ADU or JADU application ministerially, without discretionary review or a hearing, within 60 days, or the application is deemed approved. One off-street parking space is required per ADU unless an exemption applies (within a half-mile of transit, within the existing home or accessory structure, and others). Detached ADUs must connect to an approved sewer or an Environmental Health-approved on-site septic system, and ADUs may not be used as short-term rentals under 30 days.