Unincorporated Riverside County ADU rules

Maximum size

A detached ADU on a lot with a single-family home may be up to 1,600 sq ft. An attached ADU may be up to 50% of the existing home's floor area or 1,600 sq ft, whichever is less, but the 50% limit cannot stop an attached ADU of at least 1,000 sq ft. Every ADU must be at least 150 sq ft. An ADU inside existing space may add up to 150 sq ft for entry and exit. Detached ADUs on multifamily lots follow state law. Attached and detached ADUs do not count toward lot coverage.

Source: County of Riverside Ordinance No. 348.5056 (Additional Residential Accommodations), adopted text posted by the Riverside County Planning Department, Ord. No. 348, § 19.802(E), accessed 2026-11-01.

Setbacks

New ADUs need 4 ft side and rear setbacks. The front setback is the zone's, but it is reduced as far as needed so an ADU that state law allows can be built. No side or rear setback is needed for an ADU inside existing living area or an accessory structure, or for a rebuild in the same place and size. Building separation follows the Building and Fire Codes.

Source: County of Riverside Ordinance No. 348.5056 (Additional Residential Accommodations), adopted text posted by the Riverside County Planning Department, Ord. No. 348, § 19.802(G)-(H), accessed 2026-11-01.

Local ADU rules

The County's ADU rules are in Article XIXj (Additional Residential Accommodations) of Ordinance No. 348, the County Land Use Ordinance, as rewritten by Ordinance No. 348.5056. The Board of Supervisors adopted it on September 15, 2026, and it takes effect 30 days after adoption. Where the article conflicts with state law, state law governs. ADU and JADU applications are reviewed ministerially, without a hearing, under the timelines in state law (Gov. Code §§ 66317 and 66335). The text shown here is the ordinance text the Planning Department posted with the Board item; the signed copy was not read.

Source: County of Riverside Ordinance No. 348.5056 (Additional Residential Accommodations), adopted text posted by the Riverside County Planning Department, Ord. No. 348, §§ 19.801(B)(2), 19.809(A); Ord. No. 348.5056, Section 8, accessed 2026-11-01.

ADU types and how many

On a single-family lot: one new attached or detached ADU, one ADU inside the home or an accessory structure, and one JADU, in any combination state law allows (Gov. Code §§ 66314 and 66323). On a lot with an existing multifamily building: up to 8 detached ADUs, but not more than the number of existing units; with a proposed multifamily building, up to 2 detached ADUs. ADUs inside non-livable space of a multifamily building (such as storage rooms, attics or garages) are allowed up to 25% of the existing units, and at least one. Owners do not have to live on the lot.

Source: County of Riverside Ordinance No. 348.5056 (Additional Residential Accommodations), adopted text posted by the Riverside County Planning Department, Ord. No. 348, § 19.802(B), (D), accessed 2026-11-01.

Height

A detached ADU may be 16 ft. It may be 18 ft on a lot with a multistory multifamily building, or within a half mile walk of a major transit stop or high-quality transit corridor, where up to 2 more ft is allowed to match the main home's roof pitch. An attached ADU may be 25 ft or the main home's height limit, whichever is less, and no more than two stories.

Source: County of Riverside Ordinance No. 348.5056 (Additional Residential Accommodations), adopted text posted by the Riverside County Planning Department, Ord. No. 348, § 19.802(F), accessed 2026-11-01.

Parking

One space per ADU or per bedroom in the ADU, whichever is less. It may be tandem on an existing driveway or in the side or rear setback. No parking is needed within a half mile walk of public transit, in a historic district, for an ADU inside a home or accessory structure, where on-street permits are required but not offered, or when the ADU is filed with a new home and meets a state exemption. Garage, carport or parking spaces removed for an ADU do not have to be replaced. JADUs need no parking.

Source: County of Riverside Ordinance No. 348.5056 (Additional Residential Accommodations), adopted text posted by the Riverside County Planning Department, Ord. No. 348, §§ 19.802(I), 19.803(H), accessed 2026-11-01.

Junior ADUs

One JADU is allowed on a lot with a single-family home (not on multifamily lots). It must be 150 to 500 sq ft of interior livable space inside the walls of the home (an attached garage counts), with an efficiency kitchen and its own exterior door, plus an inside door to the home if it shares a bathroom. An owner must live on the lot only when the JADU shares a bathroom with the home. A deed restriction is recorded before final inspection. No parking, setback or separate utility is needed, and it may not be rented for less than 30 days.

Source: County of Riverside Ordinance No. 348.5056 (Additional Residential Accommodations), adopted text posted by the Riverside County Planning Department, Ord. No. 348, § 19.803, accessed 2026-11-01.

Utilities, sprinklers and rental

An ADU inside existing space needs no new or separate utility connection unless built with a new home. Other ADUs may need a separate connection, with fees only as state law allows. Fire sprinklers are not needed if the main home does not need them, and an ADU does not trigger sprinklers in the main home. An ADU may be rented separately but not sold separately unless state law allows it. Applications must show water and sewer service, or septic and well clearance from County Environmental Health where there is no public service.

Source: County of Riverside Ordinance No. 348.5056 (Additional Residential Accommodations), adopted text posted by the Riverside County Planning Department, Ord. No. 348, §§ 19.802(C), (K), (L)(5), 19.809(B), accessed 2026-11-01.

Existing unpermitted units

An ADU or JADU built before January 1, 2020 may be legalized under state law (Gov. Code § 66311.7).

Source: County of Riverside Ordinance No. 348.5056 (Additional Residential Accommodations), adopted text posted by the Riverside County Planning Department, Ord. No. 348, § 19.809(C), accessed 2026-11-01.

State housing department (HCD) review

On May 14, 2025, HCD found that the County's earlier ADU rules (Ordinance No. 348 as adopted April 28, 2023) did not comply with state ADU law on several points, including outdated code references, multifamily ADU counts, manufactured homes as ADUs, denial over unpermitted structures, areas where ADUs were barred, review deadlines and front setbacks. The County had until June 13, 2025 to respond. Ordinance No. 348.5056 (September 2026) rewrote these rules. No HCD letter on the new ordinance was found.

Source: HCD ordinance review letter, County of Riverside, Letter of May 14, 2025, accessed 2026-11-01.