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 implements California state ADU law (Gov. Code §§ 66310-66342) through Ordinance No. 348 (zoning) as amended for ADUs (e.g., Ord. 348.4926) and related ordinances. Riverside County also has a parallel 'Additional Residential Accommodation' (ARA) / second-unit framework. 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]
| Where ADUs Are Allowed | Residential-zoned parcels in unincorporated Riverside County with an existing or proposed single-family or multifamily dwelling |
| Max Size (Detached) | Up to 1,200 sq ft (state-law maximum) |
| Max Size (Attached) | Up to 50% of the primary dwelling's floor area (state-law standard) |
| Max Size (JADU) | 500 sq ft (within the existing single-family home) |
| Max Height | 16 ft detached; up to 18 ft near major transit or on a multifamily lot, with higher tiers in limited cases, per state law |
| Max Units | On a single-family lot: 1 ADU + 1 JADU. Multifamily lots may add more under state law (SB 1211). |
Setback & Parking Requirements [1][2]
| Rear Setback | 4 ft (state-law minimum; none for conversions of existing structures) |
| Side Setback | 4 ft (state-law minimum) |
| Front Setback | Per the underlying zoning district |
| Parking Required | 1 space per ADU, waived near transit, in conversions, and for JADUs (per state law) |
| Owner Occupancy | Not required for ADUs (state law); JADUs require owner occupancy of the home or the JADU |
| Min Lot Size | No minimum lot size to add one ADU to a qualifying single-family lot |
Permit Process & Fees [3]
| Approval Process | Ministerial building permit through the Riverside County Planning Department / Building & Safety; no discretionary hearing for a qualifying ADU |
| Review Timeline | 60-day ministerial review required by state law once a complete application is submitted |
| Permit Fees | Building/plan-check fees scale with size and valuation — confirm the current schedule with Riverside County Planning |
| Impact Fees | Waived for ADUs under 750 sq ft (state law); proportional above that, typically several thousand dollars |
| Plan Check | Included in permit; timing varies by submittal completeness |
| Pre-Approved Plans | Check Riverside County Planning for standard/pre-reviewed ADU plan options |
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Construction Costs [4]
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 [5]
| 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 implements ADUs through Ordinance No. 348 as amended (e.g., Ord. 348.4926) and related ordinances, kept consistent with California state ADU law
- California SB 1211 (in effect): up to 8 detached ADUs allowed on qualifying multifamily lots statewide
- AB 1154 (Jan 2026): owner-occupancy requirement removed for JADUs statewide
- SB 543 (Jan 2026): an ADU application is deemed approved if the County fails to act within 60 days
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.
- Rural and wine-country parcels may need septic capacity, well water, and fire-hazard (WUI) compliance — these drive cost and timeline more than the ADU itself.
- Keeping a first ADU under 750 sq ft avoids development impact fees entirely.
- Garage or barn conversions that keep the existing footprint avoid setback requirements under state law.
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 pre-screened, licensed 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 Planning — Additional Residential Accommodations (ARA) / ADU Planning Department — Accessed 2026-06-28
- Riverside County Planning Department Planning Department — Accessed 2026-06-28
- Riverside County Building & Safety Planning Department — Accessed 2026-06-28
- HomeAdvisor — 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
What are the ADU rules in unincorporated Riverside County?
How much does an ADU permit cost in Riverside County?
Do I need owner-occupancy for an ADU in Riverside County?
Is my property in unincorporated Riverside County or inside a city?
How many ADUs can I build in unincorporated Riverside County?
Building in Unincorporated Riverside County?
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