Riverside | ADUs and SB 9

Riverside ADUs and SB 9: ADU Sizes, Movable ADUs, Junior ADUs and Two-Unit Projects

An accessory dwelling unit (ADU) is a second, smaller home on a lot that keeps its main home. A junior ADU is a unit of 500 square feet or less built inside the walls of the house. SB 9 (2021) is the state law that allows two main homes on a single-family lot. Riverside's ADU rules are in Chapter 19.442 of the Municipal Code and its SB 9 rules in Chapter 19.443. This page reads both and marks where the City text and state law differ. The California ADU law page covers the statutes.

Where ADUs Are Allowed

Section 19.442.020 permits ADUs, movable ADUs and junior ADUs "in all residential zones, including all multi-family and mixed-use zones that include an existing or proposed dwelling." The chapter states its own purpose: "to ensure compliance with California Government Code Sections 65852.2 65852.22 and minimize impacts to surrounding uses and properties." The state ADU rules now sit in Government Code 66310 and the sections after it, which SB 477 added effective March 25, 2024, so this page cites those section numbers.

Several general rules in Section 19.442.030 A help owners. "There shall be no minimum lot size requirement to establish an ADU, MADU or JADU." ADU floor area is left out of lot coverage. And "No actions to correct zoning nonconformities related to physical improvements are required for ADUs," so an older house that breaks a current setback rule does not have to be fixed before an ADU is added. ADUs in a historic district must also meet Title 20, the City's historic preservation code.

Riverside ADU Rules Summary

Source: Riverside Municipal Code Section 19.442.030, Supplement 33.

How Many ADUs and How Big

On a house lot, the code limits the homes to "the primary dwelling; one attached or converted ADU; one detached, new construction ADU or MADU; and one JADU." VanPlex arithmetic: that is four homes on one R-1 lot without SB 9. On a lot with apartments, the City allows "At least one ADU, but no more than 25 percent of the existing number of multi-family dwellings" inside existing buildings, using spaces such as "storage rooms, boiler rooms, passageways, attics, basements or garages." New detached ADUs are capped at two where an apartment building is proposed, and at eight where one exists, "not to exceed the number of existing units on the lot." An existing fourplex could add one converted ADU and four detached ADUs.

Size depends on the type. An attached ADU, including a conversion, may not exceed "50 percent of the existing or proposed primary dwelling floor area or 1,200 square feet, whichever is less," but that cap "shall not prevent the establishment of an ADU that is at least" 850 square feet with one bedroom or 1,000 square feet with more than one. Next to a 1,600 square foot house, half is 800 square feet, so the floor of 850 or 1,000 square feet applies. A detached ADU may reach 1,200 square feet. A junior ADU may be up to 500.

The City's Dwell Riverside program posts permit-ready plans that the Building Division has reviewed in advance, in four sizes: 746 square feet with one bedroom, 800 and 1,020 square feet with two bedrooms, and 1,200 square feet with three. Each comes in "Craftsman, Ranch, and Spanish" exterior styles. The page says both custom and permit-ready plans "go through the standard building permit review process."

Height, Setbacks and Placement

• Height. "Single-story detached ADUs and MADUs shall not exceed 20 feet in height." Two-story detached ADUs and ADUs on the second floor of an existing accessory building "shall not exceed 30 feet in height." Attached ADUs follow the zone, which is 35 feet in every R-1 zone. In the RC hillside zone a detached ADU may not exceed "20 feet and one story in height."
• Setbacks for new ADUs. A setback is the required gap between a building and a lot line. "A minimum side and rear setback of four feet shall be provided for new one-story ADUs and MADUs under 16 feet in height." For taller ADUs, the side and rear setbacks "shall follow the underlying zone." Front and street-side setbacks follow the zone.
• Conversions. "Existing structures converted to an ADU or JADU, if applicable, shall require no additional setback." A new second story added to an existing building uses the zone setbacks.
• Junior ADUs. A junior ADU must be "within the walls of the proposed or existing primary dwelling" and include cooking appliances, a food preparation counter, refrigeration and storage cabinets. It may share the bathroom with the house.
• Attached units. An attached ADU or junior ADU "must have independent exterior access separate from" the main home.

Movable ADUs on Wheels

Riverside allows a movable ADU (MADU), a small home on a wheeled chassis. Section 19.910 defines it as a complete living unit that meets "the American National Standards Institute (ANSI) 119.5 requirements or the National Fire Protection Association (NFPA) 1192" standard. Section 19.442.030 sets these rules:

• Floor area "between 150 square feet and 430 square feet."
• No MADU "between the primary dwelling and the street within any front or street side yard."
• No MADU on a lot with apartments, and none "in any Historic District, Neighborhood Conservation Area or on a lot with a designated Cultural Resources."
• The axles, tongue and hitch "shall be fully screened and hidden from view," and the support jacks sit on paving.
• No slide-outs or tip-outs, double-pane windows with trim, and a roof that matches the main house in pitch and material.
• Connection to "water, sewer and electric utilities," and no separate street address.

Where the City Text Differs From State Law

State ADU law sets minimum rights that a city ordinance cannot cut back. Where Riverside's text is stricter, the statute controls. Three places stand out:

• Setbacks for taller ADUs. Section 19.442.030 C applies zone side and rear setbacks to new ADUs over 16 feet tall, which in R-1-7000 means a 25-foot rear yard. Government Code 66314 requires that "a setback of no more than four feet from the side and rear lot lines shall be required for an accessory dwelling unit that is not converted from an existing structure," with no height condition. A two-story ADU may use the four-foot setback under the statute.
• Owner on site for a junior ADU. Section 19.442.030 F allows one junior ADU "if the primary dwelling or JADU is owner-occupied." Government Code 66333 says "Owner-occupancy shall not be required if the junior accessory dwelling unit has separate sanitation facilities." With its own bathroom, a junior ADU needs no owner on site.
• Impact fees at exactly 750 square feet. Section 19.442.030 K waives City impact fees "For ADUs under 750 square feet" and charges proportional fees "For ADUs 750 square feet and over." Government Code 66311.5 bars impact fees on an ADU "that has 750 square feet of interior livable space or less." A 750 square foot ADU pays none.

The rest of the chapter tracks the statute. Rentals must be for "terms longer than 30 days," ADUs may not be sold apart from the house except by a 501(c)(3) housing nonprofit or faith-based group working with the Housing Authority, and fire sprinklers are not required unless the main home has them. On utilities, an ADU inside the existing home needs no new connection or capacity charge, while a new detached ADU "may be subject to a connection fee or capacity charge that shall be proportionate to the burden" on the water or sewer system. The state also sets the review clock in Government Code 66317: 15 business days to decide completeness and 60 days to approve or deny when a home already stands on the lot.

SB 9 Two-Unit Projects in Riverside

Chapter 19.443 sets the rules for a two-unit development, the SB 9 route that puts two main homes on one single-family lot under Government Code 65852.21. The City adopted it by Ordinance 7592 in 2022 and amended it by Ordinance 7744, which on November 4, 2025 repealed the former noticing section. Applications "shall be considered ministerially, without discretionary review or a hearing." The location limits match the lot split list: a single-family zone, and outside fire, flood, arroyo, habitat, hazardous waste and historic areas.

Source: Riverside Municipal Code Sections 19.443.020 to 19.443.080.

The unit count is the key number. "The maximum number of attached or detached primary dwelling units permitted on any lot in a single-family zone is two," and "No more than four total dwelling units, inclusive of up to two detached, attached, and junior ADUs" may be built on an undivided lot. That sentence cites "Chapter 4.42," a typing error for Chapter 19.442. After a lot split, each new lot holds two homes of any kind, four in total across both lots.

Design rules apply. New homes on a lot with a house must "match the existing dominant roof pitch, paint color and exterior finish materials," and front and street-side yards must be landscaped before final occupancy. The City also says "The application of any development standard that would physically prevent the development of up to two primary dwelling units or that would physically preclude either of the two units from being at least 800 square fee in floor area shall be waived." The City's SB 9 checklist words this as "at least one home of at least 800 square feet." The code and statute protect both homes, and they control.

On parking, the checklist reads the transit exception as "within one-half mile of Magnolia/Market/University Corridor or within one block of a car share vehicle." The code uses the state terms, a high-quality transit corridor or major transit stop, so a lot near another qualifying stop may also be exempt. The owner of a two-unit lot that was not created by a split "shall reside in one of the units as their principal residence," backed by a deed restriction. State law also gives the City 60 days to decide a complete application, or it is deemed approved.

Where to Go Next

Related on VanPlex: the Riverside overview, California ADU law and SB 9 in California.

Frequently Asked Questions

Official sources (checked October 2026)