Building an ADU in unincorporated Riverside County

Unincorporated areas only

Riverside County's ADU rules, in Article XIXj of Ordinance No. 348 (the county's land use ordinance), apply only within the unincorporated areas of the county. If your property is inside a city, follow that city's rules; see our City of Riverside page. For the statewide rules every city and county must follow, see our California page.

The county calls ADUs, junior ADUs and similar units "additional residential accommodations." Besides ADUs and JADUs, the county's rules cover SB 9 units, second units, guest quarters (no kitchen), and ranchet and Multiple Owner Group units, which are limited to the Eastern Coachella Valley. [1][2]

New rules in October 2026

The Board of Supervisors adopted Ordinance No. 348.5056 on September 15, 2026, rewriting the county's ADU article. The ordinance says it takes effect 30 days after adoption, which falls around October 15, 2026. Until then, the county's earlier ADU rules remain in effect; the rules on this page are the new ones. The county says the update aligns its rules with state housing law.

The rewrite follows a May 2025 review by the state housing department (HCD), which found that the earlier county ordinance did not comply with state ADU law in many ways. Among them were a 16 ft height limit that also applied to attached ADUs, an owner-occupancy rule for ADUs permitted after June 1, 2025, a parking requirement for interior ADUs in new structures, and a 50% size cap on JADUs. The new ordinance allows attached ADUs up to 25 ft, says owners don't have to live on site, exempts ADUs inside a proposed or existing house or accessory building from parking, and caps JADUs at 500 sq ft without a percentage limit. If you apply before the new rules take effect, or already have an application in, ask the Planning Department which version applies to your project.

The county's ARA page has a draft of the ordinance and hearing materials, and says eligibility checklists and a handbook are coming soon. For the adopted text, ask the Planning Department. [1][2][3][4]

What you can build

Single-family lots. On a lot zoned for single-family use, or any zone that allows a one-family home, with an existing or proposed house, you can have:

• one new attached or detached ADU,
• one ADU converted from space in the house or an accessory building, and
• ADUs and JADUs in any other combination state law allows, such as one detached ADU, one conversion ADU and one junior ADU.

Multifamily lots. On a lot with an existing apartment building, you can add up to eight detached ADUs, but no more than the number of existing units. On a lot with a proposed apartment building, you can add up to two. You can also convert non-livable space (storage rooms, attics, basements, garages and similar) into ADUs, up to 25% of the existing units, with at least one allowed.

Manufactured homes. An ADU can be an efficiency unit or a manufactured home. [1]

Size, height and setbacks

Size. An ADU must be at least 150 sq ft.

• Detached ADU on a single-family lot: up to 1,600 sq ft.
• Attached ADU: up to 50% of the house's floor area or 1,600 sq ft, whichever is less, but the 50% rule can't stop an attached ADU of at least 1,000 sq ft.
• ADU inside existing space: may add up to 150 sq ft only for an entrance.

Attached and detached ADUs don't count toward the lot coverage limit; an interior ADU counts as part of the house.

Height. A detached ADU can be 16 ft. It can be 18 ft on a lot with a multifamily, multistory building, or within a half mile walking distance of a major transit stop or high-quality transit corridor, plus up to 2 ft more there to match the house's roof pitch. An attached ADU can be 25 ft or the house's height limit, whichever is less, and up to two stories, including as a second-floor addition.

Setbacks. Side and rear: 4 ft. No side or rear setback is needed for an ADU inside existing living space or an accessory building, or for a same-size rebuild in the same spot. Front: the zone's front setback, reduced as needed to fit an ADU that state law allows. Building separation follows the building and fire codes. [1]

Parking

The county requires one parking space per ADU or per bedroom, whichever is less. It can be tandem on an existing driveway or in the rear or side setback.

No parking is required when the ADU:

• is within a half mile walking distance of public transit,
• is in an architecturally and historically significant historic district,
• is inside the house or an accessory building,
• is in an area where on-street parking permits are required but not offered to the occupant, or
• is applied for together with a new house or apartment building and meets one of the state's other exemptions.

State law also bars parking requirements when a car share vehicle is located within one block of the ADU. If a garage, carport or parking space is torn down or converted to build an ADU, you don't have to replace it. JADUs need no parking. [1][5]

Fees, utilities and timeline

Impact fees. State law bars impact fees on an ADU with 750 sq ft of interior livable space or less, or a JADU of 500 sq ft or less; on larger units, impact fees must be charged in proportion to the unit's size compared with the main house. ADUs and JADUs under 500 sq ft of interior livable space are not subject to school impact fees.

Utilities. A conversion ADU doesn't need a new or separate utility connection unless it is built at the same time as a new house. Other ADUs may need one, and the county says connection requirements and fees can be imposed only as state law allows. Where there's no public sewer or water, septic systems and wells must meet the county Department of Environmental Health's objective requirements; where there is, you must show the provider can serve the ADU.

Fire sprinklers. An ADU doesn't need sprinklers if the house doesn't, and building an ADU won't require adding sprinklers to the existing house.

Timeline. ADU applications are reviewed ministerially, without a discretionary land use permit, following state timelines. Under state law, the county must tell you within 15 business days whether your application is complete, and approve or deny a complete application within 60 days. See our ADU permit process guide and ADU cost guide. [1][5]

Junior ADUs and conversions

Junior ADUs. One JADU is allowed on a lot with an existing or proposed single-family home; JADUs aren't allowed on lots with multifamily buildings. A JADU must be 150 to 500 sq ft of interior livable space, entirely within the walls of the house; an attached garage counts as part of the house. It can't be expanded beyond the house's existing dimensions. It needs an efficiency kitchen and its own exterior door, and if it shares a bathroom it must also have interior access to the house. If it shares a bathroom, the owner must live in the house or the JADU. Before the permit is finaled, you record a deed restriction that bars selling it separately. See our junior ADU guide.

Converting a garage or other building. An ADU created inside existing space needs no side or rear setback and can add up to 150 sq ft for an entrance. A demolition permit for a detached garage being replaced by an ADU is reviewed together with the ADU application. You don't have to replace the lost parking. See our garage conversion guide.

Unpermitted units. A structure built before January 1, 2020 that meets the state definition of an ADU or JADU can be legalized under state law. [1]

Renting it out or selling it

You don't have to live on the property to have an ADU, and an ADU can be rented separately from the main house. A JADU can't be rented for periods of less than 30 days. An ADU can't be sold separately from the main house except as state law requires, and a JADU can never be sold separately. For short-term rental rules that apply to an ADU, confirm with the Planning Department.

SB 9. The new ordinance treats SB 9 units as a separate housing type and applies its SB 9 rules only to the extent state law requires. See our SB 9 guide. [1]

Permit Ready ADU plans

The county's Building and Safety Department offers four free standard ADU plans; you still pay for submission, construction and site-specific details. All are single story: a 499 sq ft studio, a 625 sq ft one bedroom, a 749 sq ft two bedroom with one bath, and a 909 sq ft two bedroom with two baths, in Traditional, Craftsman and Spanish styles.

To use them, your lot must be zoned for single-family use and have or plan a single-family home. It can't be in a flood hazard area unless County Flood Control pre-approves the plans, and it can't be in an Environmental Constraint Area. Check your lot on Map My County. Details are on the Permit Ready ADU page. [6]

Permit steps in Riverside County

1. Confirm you are in the unincorporated county

   The county's rules apply only outside city limits. Find your parcel on Map My County. [1][6]

2. Check site requirements

   Check which agency requirements apply to your lot: Environmental Health for a septic system or well, your water and sewer provider, and, depending on location, Flood Control, the Airport Land Use Commission or the Fire Department. [1]

3. Choose plans

   Use a free Permit Ready ADU plan with a preliminary site plan, completed cover sheet and the Hold Harmless Agreement and Permit Ready Clearance Form, or have custom plans drawn. [6]

4. Apply online

   Apply for a building permit through the county's PLUS Online portal and upload your plans and forms. [6]

5. Meet review conditions

   Each department reviews the application; you get an email and download a Progress report listing conditions to meet. [6]

6. Pay fees and get the permit

   Once all conditions are met and fees are paid, the permit is issued and you can download the approved plans and start building. [6]

Pre-approved and sample plans

• Permit Ready ADU (BRS-PRADU)

  Four free single-story standard ADU plans (499 to 909 sq ft) for single-family-zoned lots in unincorporated Riverside County outside flood hazard and environmental constraint areas. [6]

Who to call

• Riverside County Planning Department

  ADU zoning rules and the 2026 ordinance (Additional Residential Accommodations)

  (951) 955-3200

  4080 Lemon Street, Riverside, CA 92501

  [4]

• Riverside County Building and Safety (Land Use)

  Permit Ready ADU plans and building permits (email Landuseinfo@rivco.org)

  951-955-1800 Ext 5

  4080 Lemon St, 9th Floor, Riverside, CA 92502

  [6]

Questions people ask