Riverside Allows a Third Kind of Backyard Home

A contractor tells you the backyard unit will take eight months and a foundation. A neighbour tells you theirs arrived on a trailer. Both are describing something Riverside permits — the city recognises a category most California cities have no word for, and it comes with its own size range, its own siting rules, and a requirement to hide the axles.

Riverside allows three kinds of extra home: an accessory dwelling unit, a junior unit inside the house, and a movable accessory dwelling unit of between 150 and 430 square feet. All three are permitted in every residential zone, including multifamily and mixed-use zones with an existing or proposed dwelling (§ 19.442.020). A single-family lot can end up with the house plus three more units.

Key numbers

How many units a lot can hold

On a lot with an existing or proposed single-family residence, the code permits the primary dwelling, one attached or converted unit, one detached new-construction unit or movable unit, and one junior unit — four dwellings in total, if each meets its own standards.

On a lot with multifamily dwellings the arithmetic changes:

• Conversions of existing structures: at least one unit, and no more than 25% of the existing number of multifamily dwellings. Storage rooms, boiler rooms, passageways, attics, basements and garages all qualify, provided the result meets building standards for dwellings.
• New detached construction: no more than two on a lot with a proposed multifamily dwelling, and no more than eight on a lot with an existing multifamily dwelling — capped at the number of existing units.
• Movable units are not permitted on multifamily lots at all.

The movable unit, in detail

This is Riverside's distinctive category, and the rules are unusually physical. A movable unit must measure between 150 and 430 square feet, and:

• It must not sit between the primary dwelling and the street in any front or street-side yard.
• It gets no separate street address from the primary dwelling.
• Its undercarriage — axles, tongue and hitch — must be fully screened and hidden from view, and the levelling or support jacks must sit on a paved surface.
• Mechanical equipment must be incorporated into the structure, not placed on the roof.
• Movable units are not permitted in any Historic District or Neighborhood Conservation Area, or on a lot with designated Cultural Resources.

Ordinary accessory and junior units in a historic district are treated differently — they are allowed, subject to state accessory-unit law and the city's own historic preservation title.

Setbacks follow height

Riverside ties the side and rear setback to how tall you build.

A minimum side and rear setback of four feet shall be provided for new one-story ADUs and MADUs under 16 feet in height.

Above 16 feet, the side and rear setbacks revert to the underlying zone's requirements for the primary dwelling. Front and street-side setbacks always follow the underlying zone. A unit on the second floor of a new structure follows the zone as well.

Conversions get the best deal: an existing structure converted to a unit requires no additional setback, and a second-storey unit needs none either where the second storey already exists. Add a new second storey and the underlying zone's setbacks apply. In every case, side and rear setbacks must be sufficient for fire and safety.

Rules that remove the usual obstacles

Three provisions clear away objections that stop projects in other cities:

• No minimum lot size to establish any of the three unit types.
• The floor area of a unit is not counted when calculating lot coverage — so a lot already near its coverage limit is not disqualified.
• No actions to correct zoning nonconformities related to physical improvements are required as a condition of the unit.

Parking is equally clear: none is required for any of the three unit types, and no replacement parking is required for the house when a garage, carport or covered parking is converted.

Owner occupancy, renting and selling

Owner occupancy. On a lot with a primary dwelling and an accessory or movable unit, neither has to be owner-occupied. A junior unit is the exception: it is allowed where the primary dwelling or the junior unit is owner-occupied, and that condition is recorded as a deed restriction.

Renting. All three unit types, when rented, must be let for terms longer than 30 days. Short-stay letting is out.

Selling. A unit may not be sold or conveyed separately from the primary residence, with one narrow exception: a primary dwelling and unit developed by an IRS-recognised 501(c)(3) housing-related non-profit, or a faith-based organisation working with the Housing Authority, whose mission is to provide units to low-income households. Junior units carry a recorded deed restriction prohibiting separate sale outright.

What this article does not cover

This covers unit types, counts, sizes, setbacks and the conditions attached. It does not give Riverside's fee schedule, plan-check timeline, or the two-unit development chapter that governs lot-split projects, which sets its own unit counts. Note also that a small detached shed is a different thing entirely — single-storey accessory buildings used for tools or storage may be exempt from a building permit where the floor area is under 120 square feet and the height under 10 feet (§ 19.440.020). For the local picture see the Riverside ADU guide and the Riverside jurisdiction page.

Sources

• Riverside Municipal Code § 19.442.020 and § 19.442.030 — Accessory dwelling units: applicability and requirements
• Riverside Municipal Code § 19.440.020 — Accessory buildings and structures
• Government Code § 66323 — Ministerially approved accessory dwelling units