ADU rules in Riverside County, CA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
In unincorporated Riverside County, ADUs and Junior ADUs are governed by Ordinance No. 348, Article XIXj. Detached ADUs are capped at 1,000 sq ft and 16 feet, with 4-foot side and rear setbacks and ministerial (no-hearing) approval. State ADU Law (Gov. Code 66310-66342) overrides any stricter county rules.
Riverside County regulates accessory dwelling units in unincorporated areas through Ordinance No. 348, Article XIXj ("Additional Residential Accommodations"), most recently amended by Ord. No. 348.4997. A detached ADU may not exceed 1,000 square feet of floor area (Sec. 19.807 D.2) and a maximum height of 16 feet (Sec. 19.807 E.2). An attached or interior ADU is limited to 50% of the primary dwelling's floor area, with an 800 sq ft floor for that calculation and a 1,000 sq ft ceiling (Sec. 19.807 D.1). A Junior ADU (JADU) must be created within the existing one-family dwelling and may not exceed 500 square feet (Sec. 19.807 D.3). Detached and attached ADUs require side and rear setbacks of no less than 4 feet (Sec. 19.807 C.2). One parking space per unit is required unless waived for proximity to transit, historic districts, or car-share areas; no replacement parking is required when converting a garage or carport (Sec. 19.807 F). Importantly, California State ADU Law (Gov. Code 66310-66342) preempts local rules: in a May 14, 2025 review, HCD found several county provisions non-compliant - including owner-occupancy requirements, a 16-foot cap that fails to allow 18 feet near transit, and parking requirements for conversions - and directed the County to amend or override them. ADUs meeting Gov. Code 66323 are approved ministerially.
Building an ADU without a ministerial permit, or exceeding size/height/setback standards, can trigger code-enforcement citations and a stop-work order. Where the county standard is stricter than State ADU Law (Gov. Code 66310-66342), the state standard controls and the county cannot deny a conforming ADU.
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