These rules apply to the UNINCORPORATED areas of Riverside County — land outside any incorporated city, regulated by the Riverside County Planning Department, not a city. The County's ADU rules sit in Ordinance No. 348, Article XIXj ('Additional Residential Accommodations', Sections 19.800-19.812), last substantively amended by Ord. 348.4926 in 2020. Article XIXj expressly does NOT apply to an ADU or JADU that qualifies under the state's by-right category (Gov. Code § 66323, formerly § 65852.2(e)(1)) — those must be approved ministerially under state standards, which are more permissive than the County's on height and size. If your parcel is inside a city such as Riverside, Moreno Valley, Corona, Temecula, or Palm Springs, that city's rules apply instead of the County's.
Zoning & Size Requirements [1][2][3]
Yes — unincorporated Riverside County allows ADUs. A detached ADU can typically be up to 1,000 sq ft under California state law and the County's zoning code.
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| Where ADUs Are Allowed | By-right on lots zoned for one-family or multiple-family dwellings in unincorporated Riverside County (§ 19.803(A)). Not permitted where the existing dwelling lacks required building permits, or where the lot is constrained by water availability, water quality, or sewage disposal |
| Max Size (Detached) | 1,000 sq ft (Ord. 348 § 19.807(D)(2)). No minimum — an ADU may be an efficiency unit |
| Max Size (Attached) | 50% of the primary dwelling's floor area, but never less than 800 sq ft and never more than 1,000 sq ft (§ 19.807(D)(1)). Note state law (Gov. Code § 66321(b)(2)) bars a cap below 850 sq ft, so the County's 800 sq ft floor is below the state minimum — confirm which applies before you design to 800 |
| Max Size (JADU) | 500 sq ft, within the existing one-family dwelling or an attached structure, plus up to 150 sq ft of expansion for ingress/egress (§ 19.807(D)(3)) |
| Max Height | 16 ft for both attached and detached ADUs under the County ordinance (§ 19.807(E)(2)) — but state law requires more in three cases: 18 ft detached within half a mile of a major transit stop or high-quality transit corridor, 18 ft detached on a lot with a multistory multifamily dwelling, and 25 ft (or the zone's limit for the primary dwelling, whichever is lower) for an attached ADU (Gov. Code § 66321(b)(4), as amended by SB 543). State law also requires an extra 2 ft to match the primary dwelling's roof pitch |
| Max Units | Under state law (Gov. Code § 66323) a single-family lot gets one ADU plus one JADU approved ministerially, and Article XIXj does not apply to those. For anything outside that state category, the County's own table allows one attached OR interior OR detached unit per one-family lot — § 19.803(B) says the allowances apply 'but not a combination thereof'. Multifamily lots: one interior ADU or 25% of existing units, whichever is greater, plus two detached ADUs |
Setback & Parking Requirements [1][2][3]
Unincorporated Riverside County requires a rear setback of about 4 ft for a detached ADU, and no additional parking is required. Owner-occupancy is set by local rules (confirm with the County).
| Rear Setback | 4 ft minimum for attached and detached ADUs (§ 19.807(C)(2)); interior ADUs and JADUs follow the underlying zone, with 4 ft required for any egress expansion |
| Side Setback | 4 ft minimum for attached and detached ADUs (§ 19.807(C)(2)) |
| Front Setback | Per the underlying zoning district (§ 19.807(C)(2)) |
| Parking Required | 1 regular or tandem space for a detached, attached, or new-structure interior ADU. No parking for a JADU, a guest quarter, or an interior ADU in an existing structure. Waived entirely within half a mile walking distance of transit, in a historic district, in a permit-parking area where the new unit is not issued permits, or within 1 mile of a car-share area. No replacement parking when a garage or carport becomes an ADU (§ 19.807(F)) |
| Owner Occupancy | The County ordinance requires the owner to live in the primary dwelling or the ADU for any ADU permitted after June 1, 2025 (§ 19.808(A)(1)). State law says otherwise: Gov. Code § 66315 bars a local agency from imposing 'an owner-occupant requirement' on an ADU. The County's own § 19.812(B) defers to state law where the two conflict, and this provision is part of the ordinance HCD has under review — raise it at application rather than assuming you must occupy. JADUs are different: § 66333(b) still allows an owner-occupancy requirement where the JADU shares a bathroom with the house |
| Min Lot Size | No minimum lot size for an ADU (the County's 7,200 sq ft minimum applies only to detached Second Units and Guest Quarters, which are not ADUs — § 19.807(A)(1)). On septic, the binding constraint is density, not zoning: see the septic note below |
Permit Process & Fees [4][5][6][7]
An ADU in unincorporated Riverside County is approved administratively — no public hearing — within 60 days to act on an ADU or JADU application once submitted (§ 19.804(C)). Under SB 543 the County must also tell you within 15 business days whether your application is complete, and an application is deemed complete if it misses that deadline (Gov. Code § 66317(a)) — that rule is state law and is not written into Article XIXj; permit fees are County Building & Safety charges a deposit, not a fixed fee: $6,472.45 for an ADU (BRS-ADU) or a Permit Ready ADU, $3,601.37 for a JADU, $1,757.52 for a production-tract ADU (Form 284-035, rev. July 2026). Final cost is calculated at application, and surcharges apply on top. Environmental Health charges separately — roughly $796 for a percolation evaluation or a new conventional septic system, $1,393 for an advanced treatment system. The Planning Department has no ADU-specific fee.
| Approval Process | Ministerial. § 19.804(A): 'No discretionary land use permit such as, but not limited to, a plot plan or conditional use permit is required.' But § 19.805(A) requires approvals from the Department of Environmental Health, the Fire Department, and the Airport Land Use Commission to be obtained BEFORE you submit, plus written DEH confirmation for the septic system and sign-off from the water and sewer purveyors |
| Review Timeline | 60 days to act on an ADU or JADU application once submitted (§ 19.804(C)). Under SB 543 the County must also tell you within 15 business days whether your application is complete, and an application is deemed complete if it misses that deadline (Gov. Code § 66317(a)) — that rule is state law and is not written into Article XIXj |
| Permit Fees | County Building & Safety charges a deposit, not a fixed fee: $6,472.45 for an ADU (BRS-ADU) or a Permit Ready ADU, $3,601.37 for a JADU, $1,757.52 for a production-tract ADU (Form 284-035, rev. July 2026). Final cost is calculated at application, and surcharges apply on top. Environmental Health charges separately — roughly $796 for a percolation evaluation or a new conventional septic system, $1,393 for an advanced treatment system. The Planning Department has no ADU-specific fee |
| Impact Fees | None on an ADU of 750 sq ft or less of interior livable space, or a JADU of 500 sq ft or less (Gov. Code § 66311.5(c)(1)); charged proportionately to the primary dwelling's floor area above that. Riverside's own development impact fee ordinance (Ord. 659) does exempt second units, but by cross-reference to Ordinance 348 sections that have since been rescinded or reused — confirm the exemption in writing at application rather than relying on it |
| Plan Check | Included in permit; timing varies by submittal completeness |
| Pre-Approved Plans | Yes — the Permit Ready ADU program (BRS-PRADU) offers four free standard plans at 499, 625, 749 and 909 sq ft. Three of the four sit under the 750 sq ft impact-fee threshold, and the 749 sq ft plan is one square foot under it. The plans are free; the permit deposit is the same as a custom ADU. Not available if the parcel is in a Flood Hazard Area (needs Riverside County Flood Control pre-approval) or an Environmental Constraint Area |
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Construction Costs [8]
Building an ADU in unincorporated Riverside County typically costs $140,000-$320,000 ($200-$400 per sq ft); a garage conversion runs $70,000-$160,000.
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Construction costs in unincorporated Riverside County mirror the surrounding Inland Empire market. Rural parcels can cost more due to septic, well, access, and fire-hazard (WUI) requirements, so treat these as planning ranges and get local bids.
| Average Build Cost | $140,000-$320,000 |
| Cost per Sq Ft | $200-$400 |
| Garage Conversion | $70,000-$160,000 |
| Prefab ADU (Installed) | $100,000-$240,000 |
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Rental Income Potential [9]
| Avg Rent (1 Bedroom) | $1,500-$2,100/mo |
| Avg Rent (2 Bedroom) | $1,900-$2,700/mo |
| Short-Term Rental | Subject to County short-term rental rules — confirm with Riverside County Planning |
| Est. Annual ROI | 6-9% annual return |
Recent Law Changes
What's Changed Recently
- Riverside County's adopted ADU rules are Ordinance No. 348, Article XIXj (§§ 19.800-19.812), last substantively amended by Ord. 348.4926 effective September 24, 2020 — they predate most of the current state ADU statutes and still cite Gov. Code §§ 65852.2 and 65852.22, which SB 477 repealed in 2024
- A rewrite is pending: Change of Zone No. 2600012 would reorganize Article XIXj, add SB 9 units, and update development standards. The Planning Commission voted 4-0 on July 8, 2026 to recommend the Board of Supervisors tentatively approve it. No Board hearing date has been set, so the 2020 standards remain in force
- California SB 1211 (in effect): up to 8 detached ADUs allowed on qualifying multifamily lots statewide
- AB 1154 (Jan 2026): JADU owner-occupancy narrowed — a local agency may require it only where the JADU shares a bathroom with the main house (Gov. Code § 66333(b))
- SB 543 (Jan 2026): an ADU application is deemed approved if the County fails to act within 60 days, and deemed complete if the County misses the 15-business-day completeness review
Tips for Building an ADU in Unincorporated Riverside County
- Confirm your parcel is unincorporated before relying on County rules — addresses inside cities like Riverside, Corona, Moreno Valley, or Temecula follow that city's ADU code. Sun City and Quail Valley are now Menifee; Rubidoux, Mira Loma, Glen Avon and Pedley are now Jurupa Valley.
- On septic, lot size decides everything. County Environmental Health requires half an acre (gross) per dwelling unit — house plus ADU counts as two — or you must install an advanced treatment system, and every structure on the property has to connect to it. Below a quarter acre per unit, no septic system is permitted at all, so a small unsewered parcel cannot take an ADU.
- Some unincorporated areas cannot take a new septic system at all, which stops an ADU outright rather than just adding cost. Cherry Valley (Ord. 871) and Quail Valley (Ord. 856) both bar expanding or modifying an existing system to serve new construction — which is exactly what adding an ADU does. New systems have been prohibited in Homeland/Romoland since 1982, and on parcels under half an acre over the Mission Creek or Desert Hot Springs aquifers where sewer is available. Check your area before you spend anything.
- On a private well, the well itself has to pass. Ordinance 682 requires 1 gallon per minute per dwelling sustained for four hours without pumping dry — so a house plus an ADU means proving 2 gpm — and the rule explicitly reaches existing wells on a property being improved, not just new ones. Testing must be supervised by a licensed C-57, C-61 or D-21 contractor or a certified hydrogeologist.
- An ADU does not need fire sprinklers unless the main house has them — that is state law and the County ordinance agrees. But the County's own ADU plan set adds two separate triggers that catch rural parcels: sprinklers may be required if fire flow is insufficient, or if the nearest hydrant is more than 400 feet from the furthest point of the ADU measured along access roads, driveways and exterior walls.
- Line up Environmental Health, Fire Department and Airport Land Use Commission approvals before you submit — § 19.805 requires them in hand, and a percolation test certified within the last five years (or recertified within ten). This is the step that adds months, not the plan check.
- Keeping an ADU at 750 sq ft or less of interior livable space avoids development impact fees entirely under Gov. Code § 66311.5.
- The County publishes free Permit Ready ADU plans. They do not reduce the permit deposit, but they cut design cost and plan-check risk — as long as your parcel is outside a Flood Hazard Area and an Environmental Constraint Area.
ADU Builders in Riverside County
Looking for an ADU builder in Riverside County? ADU projects need a contractor experienced with local zoning, the permit process, and utility connections — not every general contractor has built one. We match Riverside County homeowners with experienced local ADU contractors who provide free, no-obligation quotes. Getting at least three bids typically saves 20-40% on the same scope.
Cities Within Riverside County
If your property is inside one of these incorporated cities, follow that city's ADU code — not the County rules above:
Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Riverside County Ordinance No. 348, Article XIXj — Additional Residential Accommodations (§§ 19.800-19.812) Municipal Code — Accessed 2026-08-14
- Riverside County Planning — Additional Residential Accommodations (ARA) / ADU Planning Department — Accessed 2026-08-14
- California Gov. Code §§ 66315, 66321, 66323, 66333 — ADU state standards State Law — Accessed 2026-08-14
- Riverside County Building & Safety — Fixed Fees (Form 284-035, rev. 7/2026) Planning Department — Accessed 2026-08-14
- Riverside County Building & Safety — Permit Ready ADU Planning Department — Accessed 2026-08-14
- 2024 Riverside County Local Agency Management Program (OWTS/septic density standards and prohibition areas) Government Data — Accessed 2026-08-14
- Riverside County Ordinance No. 682.6 — water wells (§ 16, minimum well production) Municipal Code — Accessed 2026-08-14
- Angi — ADU Construction Cost Data Market Data — Accessed 2026-06-28
- HUD Fair Market Rents Government Data — Accessed 2026-06-28
- California ADU Handbook — HCD Government Data — Accessed 2026-06-28
Frequently Asked Questions
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