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Riverside County regulates ADUs on parcels in the unincorporated county through Title 17 (Zoning) of the Riverside County Code of Ordinances, primarily Ordinance No. 348 as amended, with the current ADU framework embodied in Ordinance No. 348.4890 and successor amendments. The county's ADU standards are a thin local overlay on top of California Government Code sections 65852.2 (ADU) and 65852.22 (JADU), which statewide preempt most local ADU discretion. The county ordinance has been amended repeatedly to track the state's rolling ADU reform package: AB 68 / AB 881 / SB 13 (2019), AB 976 (2019 owner-occupancy sunset), AB 2221 / SB 897 (2022 review-timeline and design-standard clarifications), AB 1033 (2023 condo-ADU optional program), AB 2533 (2024 code-compliance amnesty for older ADUs), and SB 543 (2024 further design-review constraints). Riverside County permits up to one ADU plus one JADU per single-family parcel by right, and up to the state-mandated count on multifamily lots; parking is not required within 1/2 mile of a public transit stop. The county's locally-controlled contributions on top of state law are the Fire Safety / Fire Protection Area standards for ADUs in Very High Fire Hazard Severity Zones (VHFHSZ) around Palomar Mountain, Idyllwild / San Jacinto Mountains, San Bernardino National Forest edges, Anza, Aguanga, and the back-country of East County; the Coachella Valley Multiple Species Habitat Conservation Plan (CVMSHCP) mitigation-fee and site-review regime; Airport Land Use Compatibility (ALUC) overlays around March Air Reserve Base, Riverside Municipal, French Valley, Hemet-Ryan, Flabob, Banning Municipal, Bermuda Dunes, Blythe, Chiriaco Summit, Desert Center, Jacqueline Cochran Regional (Thermal), and Perris Valley (Skylark / Perris); and Hillside / Slope Ordinance design review in hillside areas. As of 2026-04-20, Riverside County has NOT opted into the AB 1033 condominium-separation program for unincorporated ADUs, so ADUs cannot presently be sold separately from the primary dwelling.

• Riverside County Ordinance No. 348 (Zoning Ordinance) — Accessory Dwelling Unit provisions
• Riverside County Planning Department ADU applicant information and handouts
• Ordinance No. 348.4890 (approx) — 2020 ADU ordinance conforming to AB 68 / AB 881 / SB 13
• Ordinance No. 348 amendments tracking state law (2022-2024)

State-floor overlay: California state law (Gov. Code 65852.2, 65852.22) preempts most local ADU regulation statewide. The state mandates ministerial approval, caps local fees, requires 60-day permit review from a complete application, forbids local owner-occupancy requirements on full ADUs (owner-occupancy remains permissible for JADUs only), sets minimum allowed sizes (850 sqft one-bedroom, 1000 sqft two-bedroom floor), forbids parking requirements within 1/2 mile of transit or on replacement-covered-parking ADUs, caps impact fees at zero for ADUs under 750 sqft, and via AB 1033 (2023) permits local opt-in for condominium-separation (Riverside County has not opted in). Riverside County's ordinance reiterates and applies these floors, adding only locally-controlled overlays: fire-hardening (Chapter 7A WUI), airport compatibility (ALUC), habitat-conservation mitigation (CVMSHCP), and hillside grading. Where a project is in a VHFHSZ or CVMSHCP area, state ADU preemption still applies to the ADU allowance itself but does not preempt the county's separate fire and habitat authority over site design.

Riverside County administers or co-administers several overlay regimes that materially affect ADU siting on unincorporated parcels. The scale is unusually large because the county spans 7,200+ square miles of widely varying geography: (1) Very High Fire Hazard Severity Zones (VHFHSZ), designated by CAL FIRE and the State Board of Forestry and reviewed by the Riverside County Fire Department under CAL FIRE contract, cover very large portions of the mountain back-country — Palomar Mountain extensions, San Jacinto Mountains (Idyllwild, Pine Cove, Mountain Center), Santa Rosa Mountains, Thomas Mountain, Anza-Borrego flank, and the eastern San Bernardino National Forest / Little San Bernardino Mountains edge — and drive Chapter 7A WUI ignition-resistant construction, Pub. Res. Code 4291 defensible space, and access / water-supply standards; (2) the Coachella Valley Multiple Species Habitat Conservation Plan (CVMSHCP), administered by the Coachella Valley Conservation Commission (CVCC) and overseen by the California Department of Fish and Wildlife (CDFW) and USFWS, covers approximately 1.1 million acres of the Coachella Valley and imposes a Local Development Mitigation Fee (LDMF) plus overlay-specific biological review; (3) the separate Western Riverside County Multiple Species Habitat Conservation Plan (WR-MSHCP), administered by the Western Riverside County Regional Conservation Authority (RCA), covers much of western Riverside and similarly imposes an LDMF and biological review; (4) Airport Land Use Compatibility Plans (ALUCPs) administered by the Riverside County ALUC around roughly a dozen public-use and military-use airports set safety zones and noise contours; (5) FEMA Special Flood Hazard Areas (SFHA) along the Santa Ana River, San Jacinto River, Temescal Wash, Murrieta Creek, Temecula Creek, Whitewater River, Coachella Valley wash network, Palo Verde / Colorado River floodplain, and numerous desert washes require elevation-to-or-above-BFE plus county freeboard; (6) Hillside / Slope review per the county Hillside Ordinance applies in mountain and canyon settings; (7) the State Route 243 Scenic Corridor and several historic-resource overlays (Gilman Hot Springs, historic town sites) add aesthetic-review layers in their respective footprints. Seismic-retrofit overlays are not a Riverside-County-administered regime (unlike parts of Los Angeles and the Bay Area); California seismic building-code compliance applies statewide through the California Building Code adopted by the county. Coastal Commission jurisdiction does NOT reach Riverside County — the county is entirely inland.

• CAL FIRE / State Board of Forestry Very High Fire Hazard Severity Zones (VHFHSZ) and County Fire Code — Very substantial portions of unincorporated Riverside County are designated VHFHSZ in either the State Responsibility Area (SRA) or Local Responsibility Area (LRA). Principal affected areas include the San Jacinto Mountains (Idyllwild, Pine Cove, Mountain Center, Fern Valley), Palomar Mountain extensions and the Palomar / Cleveland National Forest interface, Santa Rosa Mountains, Thomas Mountain, Anza Valley / Cahuilla, Aguanga, Temescal Mountain / Cleveland National Forest flank, Lake Mathews / Gavilan Hills, the Badlands, the San Bernardino National Forest edge (Forest Falls / Angelus Oaks extensions into northern Riverside), and parts of the Santa Ana Mountains flank above Lake Elsinore and Corona. An ADU in a VHFHSZ must comply with California Building Code Chapter 7A (WUI-rated exterior materials: ignition-resistant siding, dual-pane windows, 1/8-inch-max vent screens, Class A roofing, non-combustible eaves / soffits / decks), minimum 100-foot defensible space per Pub. Res. Code 4291 (200 feet in some local jurisdictions), minimum driveway width / turnaround per Riverside County Fire Department standards, and minimum fire-flow water supply (2,500 gpm residential baseline, reduced to as low as 750 gpm for ADUs equipped with NFPA 13D residential sprinklers and served by on-site storage). The Riverside County Fire Department (operated under contract by CAL FIRE) or the local paid / volunteer FPD reviews the ADU permit. Recent fire history (2018 Cranston Fire, 2020 Apple Fire edge, 2022 Fairview Fire, multiple 2023-2025 brush starts) reinforces enforcement; rebuild surges post-fire lengthen permit backlogs in affected areas. Fire Safety Area permit fees and pre-construction / pre-inspection site visits are cost-recovery add-ons to the base county permit.
• Coachella Valley Multiple Species Habitat Conservation Plan (CVMSHCP) — The CVMSHCP is a 75-year regional habitat conservation plan covering approximately 1.1 million acres of the Coachella Valley and surrounding foothills, designed to conserve 27 listed and sensitive species (including Coachella Valley fringe-toed lizard, Peninsular bighorn sheep, desert tortoise, flat-tailed horned lizard, Coachella Valley milk-vetch, and others) while streamlining incidental-take coverage for covered activities. Participating jurisdictions are Riverside County (unincorporated), Cathedral City, Coachella, Desert Hot Springs, Indian Wells, Indio, La Quinta, Palm Desert, Palm Springs, and Rancho Mirage; the plan is administered by the Coachella Valley Conservation Commission (CVCC). An ADU on an unincorporated Coachella Valley parcel is typically a 'covered activity' receiving automatic take coverage in exchange for payment of the CVMSHCP Local Development Mitigation Fee (LDMF); the current LDMF (2026) is approximately $6,900 per new residential unit for most of the plan area, with higher rates in certain biological-conservation zones. Parcels inside Conservation Areas or Essential Ecological Processes Areas may require additional biological surveys (covered-species presence / absence, focused surveys, pre-construction clearance) even for a single ADU. The LDMF is NOT waived by SB 13's impact-fee exemption for sub-750-sqft ADUs — the fee is a habitat-conservation mitigation, not a county impact fee.
• Western Riverside County Multiple Species Habitat Conservation Plan (WR-MSHCP) — The WR-MSHCP is a 75-year regional habitat conservation plan covering approximately 1.26 million acres of western Riverside County, conserving 146 species and their habitats while providing incidental-take coverage. Participating jurisdictions are Riverside County (unincorporated western portion), Banning, Beaumont, Calimesa, Canyon Lake, Corona, Eastvale, Hemet, Jurupa Valley, Lake Elsinore, Menifee, Moreno Valley, Murrieta, Norco, Perris, Riverside, San Jacinto, Temecula, and Wildomar. Administered by the Western Riverside County Regional Conservation Authority (RCA). An ADU on an unincorporated western Riverside parcel is typically a 'covered activity' receiving automatic take coverage in exchange for payment of the WR-MSHCP Local Development Mitigation Fee; the current LDMF (2026) is approximately $2,500-$3,200 per residential unit depending on lot-density category, billed at building-permit issuance. Parcels inside Criteria Cells, Public/Quasi-Public Lands, or sensitive Rough Step habitat areas may require additional biological surveys and RCA Joint Project Review (JPR) before permit issuance — a process that can add 4-12 weeks to the ADU timeline. The LDMF is likewise not waived by the SB 13 county-impact-fee exemption.
• Airport Land Use Compatibility Plans (ALUCP) — Riverside County Airport Land Use Commission — The Riverside County ALUC (staffed by TLMA Planning) adopts and administers ALUCPs for Riverside Municipal Airport, March Air Reserve Base (federal military joint-use), French Valley Airport, Hemet-Ryan Airport, Flabob Airport, Banning Municipal Airport, Bermuda Dunes Airport, Blythe Airport, Chiriaco Summit Airport, Desert Center Airport, Jacqueline Cochran Regional Airport (Thermal), and Perris Valley Airport (Skylark). ALUCP airport influence areas (AIAs) extend 2-5+ miles beyond each airport depending on runway configuration and establish safety zones (Zones 1-6) and noise contours (60/65/70 dB CNEL). Principal ALUCP overlays affecting unincorporated parcels: March ARB AIA extends extensively across Mead Valley, Meadowbrook, Good Hope, Quail Valley, and parts of Moreno Valley / Perris fringes; Riverside Municipal AIA covers Arlington / La Sierra / Jurupa Valley fringe; French Valley AIA covers Winchester, French Valley, Temecula fringe; Jacqueline Cochran Regional AIA covers Thermal, Mecca, and North Shore / Salton Sea fringe; Hemet-Ryan AIA covers Valle Vista and Hemet fringe. An ADU in a safety zone may face density restrictions, CC&R / avigation-easement recording requirements, and noise-attenuation construction standards (STC-rated windows, forced-air HVAC with acoustic treatment). The ALUC reviews county-referred projects; in a safety-zone conflict the Board of Supervisors may override only by a super-majority vote per PUC 21676. March ARB's federal-military status adds a DoD consultation layer.
• FEMA Special Flood Hazard Areas (SFHA) — National Flood Insurance Program — The Riverside County Flood Control and Water Conservation District and TLMA Planning jointly administer FEMA NFIP floodplain regulations for unincorporated parcels. Principal SFHA extents include the Santa Ana River corridor (Jurupa Valley, Eastvale, Norco, Corona extensions), San Jacinto River and Salt Creek (San Jacinto Valley, Hemet, Valle Vista, Lakeview, Nuevo, Homeland, Winchester), Temescal Wash (Temescal Canyon, Lake Elsinore extensions), Murrieta Creek and Temecula Creek (Murrieta, Temecula, Wine Country), Pechanga Creek, Whitewater River and its Coachella Valley tributaries (Palm Springs to Mecca), Mission Creek / Morongo Wash, Long Canyon Wash, numerous desert washes feeding the Salton Sea, and the Palo Verde Valley / Colorado River floodplain. ADUs in an SFHA require lowest-floor elevation to or above Base Flood Elevation plus county freeboard (typically 1 foot), flood vents on enclosures below BFE, anchoring, and a post-construction Elevation Certificate. Desert-wash parcels additionally face county alluvial-fan and debris-flow overlay considerations not governed by FEMA. Owners should confirm the currently-effective FIRM panel at permit time; FEMA has been revising panels in multiple Riverside County watersheds on a rolling basis.
• Riverside County Hillside / Slope Ordinance — The county's Hillside / Slope Ordinance applies objective design and grading standards to parcels with slopes exceeding 10-15% (thresholds vary by zone and overlay). Affected areas include the San Jacinto Mountains, Palomar Mountain extensions, Santa Rosa Mountains, Lake Mathews / Gavilan Hills, Box Springs Mountains fringe, De Luz Heights, Temescal Canyon flanks, Lakeland Village hills, and the Anza / Cahuilla benches. An ADU proposed on a hillside lot may require a slope analysis, cut/fill limits, grading plan subject to California Building Code Chapter 18 and the county grading ordinance, erosion and sediment control per the county MS4 permit, and visual-impact mitigation (earth-tone color palette, view-corridor protection). The state ADU preemption does not displace the county's objective hillside standards; those standards apply to the ADU on equal terms with any new residential construction.
• Historic and scenic-corridor overlays — Riverside County has a limited number of designated historic properties and districts in the unincorporated area — Gilman Hot Springs, Soboba Hot Springs, selected ranches in Anza / Cahuilla, mining-era structures in the Salton Sea / Little San Bernardino Mountains area, and the historic Pinyon Pines / Mountain Center cabin-era structures. ADUs on a parcel with a county-designated historic resource must go through Certificate of Appropriateness design review in addition to the ministerial ADU permit; the review is objective but may impose material, window-type, and massing constraints to preserve historic character. State Route 243 (the Banning-Idyllwild Panoramic Highway) is a designated State Scenic Highway; parcels visible from the SR-243 corridor may face additional objective visual-mitigation standards. Neither overlay displaces the state ADU allowance itself but both add procedural and design requirements on top of the baseline ministerial permit.

Riverside County is the fourth-largest county in California by land area at approximately 7,208 square miles, spanning from the Orange County border west to the Arizona border east, and covering the Coachella Valley, Palo Verde Valley, San Jacinto Mountains, Santa Ana Mountains flank, Santa Rosa Mountains, Little San Bernardino Mountains, and the San Gorgonio Pass. Unincorporated area covers the majority of the county's land (approximately 5,500+ sqmi after the 28 incorporated cities' footprints are subtracted) and includes densely developed near-urban fringes (Mead Valley, Home Gardens, Woodcrest, Lakeland Village, Meadowbrook, Good Hope, Quail Valley, Winchester, French Valley, Wine Country / Temecula Valley wine region, Valle Vista), rural / mountain / desert communities (Anza, Aguanga, Pine Cove, Idyllwild-Pine Cove, Mountain Center, Pinyon Pines, Sage, Cahuilla, Warner Springs extensions), and tribal reservations (Cahuilla, Morongo, Agua Caliente, Soboba, Torres-Martinez, Pechanga, Santa Rosa, Augustine, Twenty-Nine Palms Band — tribal lands are not county-permitted). The 28 incorporated cities (Banning, Beaumont, Blythe, Calimesa, Canyon Lake, Cathedral City, Coachella, Corona, Desert Hot Springs, Eastvale, Hemet, Indian Wells, Indio, Jurupa Valley, La Quinta, Lake Elsinore, Menifee, Moreno Valley, Murrieta, Norco, Palm Desert, Palm Springs, Perris, Rancho Mirage, Riverside, San Jacinto, Temecula, Wildomar) permit their own ADUs independently. The Transportation and Land Management Agency (TLMA) — specifically the Planning Department and the Building & Safety Department — is the single point of contact for ADU permits on unincorporated parcels, combining zoning review, building plan check, fire-district referral (primarily Riverside County Fire Department / CAL FIRE contract), grading / drainage review, and environmental review (CEQA exemption typically applies for ministerial ADUs per Gov. Code 65852.2(f) and Pub. Res. Code 21080(b)(8)).