ADU rules in Ontario

Detached ADUs are allowed on residential lots citywide: Table 5.02-1 lists Accessory Dwelling Units as P (permitted) in every residential district (AR-2, RE-2, RE-4, LDR-5, MDR-11, MDR-18, MDR-25, HDR-45), in the MU-1 subareas and MU-3, MU-6, MU-8B, MU-8C and MU-8E, and in the AG overlay, and Ontario Development Code (ODC) 5.03.010, amended and restated in full by Urgency Ord. 3323 (adopted December 16, 2025, effective immediately), sets two tracks. Class 1 units follow Gov. Code 66323 in any residential or mixed-use district: one new detached ADU of up to 800 sq ft of livable space with 4-foot side and rear setbacks on a single-family lot (combinable with a conversion ADU and a JADU), and on multifamily lots up to two detached ADUs with a proposed building or up to eight, never more than the existing unit count, with an existing one. Class 2 units follow Gov. Code 66314-66322 on any lot zoned for single-family or multifamily use and must meet subsection F: 850 or 1,000 sq ft, one story and 16 feet, 4-foot side and rear setbacks, front setbacks of 30 feet from arterials and 20 feet from other streets, zone lot-coverage caps (30 to 60 percent on traditional single-family lots), matching materials and roof slope, window screening and evergreen landscape screening, none of which may push a unit below 800 sq ft. The main exception is the Ontario International Airport safety zones (see caveats).

Attached and conversion ADUs are allowed on the same lots. As Class 1 units, one ADU may be created within the proposed or existing space of a single-family home or within an existing accessory structure (plus up to 150 sq ft to accommodate ingress and egress) together with one JADU, and on multifamily lots at least one converted ADU, up to 25 percent of the existing units, may be built in non-livable space such as storage rooms, boiler rooms, attics, basements or garages (D.3.a(1) and (3)). Class 2 attached additions may reach 25 feet or the zone's height limit, whichever is lower, and two stories, and are capped at 850 or 1,000 sq ft of interior livable space and at 50 percent of the existing primary dwelling's floor area, but never below 800 sq ft (E.2.d, F.1). JADUs (up to 500 sq ft of interior livable space inside a single-family structure, an attached garage counting as part of it, with an efficiency kitchen) are allowed only on lots zoned for single-family residences (C.5, E.1.c); the city's ADU page adds that they are not permitted on lots with multifamily dwellings or multiple single-family dwellings or inside detached accessory structures.

For Class 2 ADUs, ODC 5.03.010.F.1 caps size at 850 sq ft of interior livable space for a studio or one-bedroom unit and 1,000 sq ft for two or more bedrooms, attached units are further limited to 50 percent of the existing primary dwelling, and the percentage cap, front setbacks, lot coverage and open-space rules may not push a unit below 800 sq ft of interior livable space (F.1.c). The Class 1 detached unit is capped at 800 sq ft of livable space (D.3.a(2)) and a JADU at 500 sq ft of interior livable space (C.5.a). Heights (E.2): 16 feet for detached units, 18 feet within half a mile walking distance of a major transit stop or high-quality transit corridor plus 2 feet to match the main roof pitch (20 feet maximum), 18 feet on a lot with a multistory multifamily building, and 25 feet (or the zone limit) and two stories for attached units, measured from existing legal grade or the lowest floor, whichever is lower. Class 2 detached units are also held to one story (F.10), a limit Ord. 3323 added; Gov. Code 66315 bars standards beyond those in 66314, and HCD's April 22, 2026 findings on San Bernardino's identically worded clause called a one-story limit unlawful where two stories fit the height limit, but HCD has not reviewed Ontario's text. The size caps sit exactly at the Gov. Code 66321(b)(2)-(3) floors as amended by SB 543, so the state's 1,200 sq ft detached default in 66314(d)(5) does not apply while the ordinance stands.

No: ODC 5.03.010.E.7.a states that ADUs are not subject to an owner-occupancy requirement, matching Gov. Code 66315. For a JADU, a natural person with legal or equitable title must live in the main home or the JADU only when the JADU shares sanitation facilities with the main home; the requirement falls away when the JADU has its own bathroom or the property is owned by another governmental agency, land trust or housing organization (E.7.b), tracking Gov. Code 66333(b) as amended by AB 1154 effective January 1, 2026. A JADU deed restriction (no separate sale, approved size and attributes) must be recorded before the JADU's certificate of occupancy (E.8). The city's ADU FAQ handout (Rev. 01.21.2025), still linked from the ADU page, prints the superseded universal JADU owner-occupancy rule.

For Class 2 ADUs, ODC 5.03.010.F.6.a requires one off-street space per ADU or per bedroom, whichever is less, which may sit in setback areas or be tandem. No space is required within half a mile walking distance of public transit (defined in C.11 to include any fixed-route, set-fare bus stop or train station), in an architecturally and historically significant historic district, for a unit that is part of the home or an accessory structure, where on-street permits are required but not offered to the occupant, within one block of a car-share stop, or when the ADU is filed with a new home and meets one of those tests (F.6.b, tracking Gov. Code 66322(a)). Subsection F does not apply to Class 1 (Gov. Code 66323) units or JADUs, and garage, carport or uncovered spaces demolished for or converted to an ADU never need replacement (F.6.c; Gov. Code 66314(d)(11)), which ODC 6.01.010.F.8 carries as an exception to its replacement-garage rule. Ord. 3323 relaxed the 2025 text (Ord. 3299), which required one space per ADU regardless of bedroom count.

Every ADU and JADU that meets subsection E (and, for Class 2 units, subsection F) is allowed with only a building permit (ODC 5.03.010.D.1): the city must decide completeness within 15 business days, approve or deny within 60 days of a complete application or the application is deemed approved, and consider ADU and JADU permits without discretionary review or a hearing, with written appeals of an incompleteness finding or a denial to the Planning Commission decided within 60 business days (D.2), tracking Gov. Code 66317 as amended by SB 543. Subsection I adds an optional conditional use permit route for an ADU that does not meet the objective standards, the same clause HCD found unlawful in its April 22, 2026 findings on San Bernardino's identical text (Gov. Code 66316 bars discretionary processes). Applications go through the Building Department's Citizen Portal Access with Planning, Engineering, Fire and utility (OMUC) review; the city offers no city-standard plans, and a private vendor's city-preapproved plan gets plan check within 30 days of a complete submittal.

ADUs and JADUs may be rented but not for a term shorter than 30 days, a ban that applies regardless of when the unit was created (ODC 5.03.010.E.4); state law independently requires terms longer than 30 days for Class 1 units (Gov. Code 66323(e)) and JADUs (66333(g)), so even a 30-day rental is out for those units. An ADU or JADU may not be sold or conveyed separately from the lot and primary dwelling except under the Gov. Code 66341 qualified-nonprofit path (E.5), and the council record from 2025 through September 15, 2026 shows no Gov. Code 66342 ADU-condominium ordinance. Citywide, short-term rentals need a short-term rental permit and business license and are limited to hosted and un-hosted stays at the host's primary residence (un-hosted stays capped at 90 days a year) in listed residential and mixed-use zones, while vacation rentals are prohibited (ODC 5.03.363.B, I). Long-term rental units over seven years old fall under the Systematic Health and Safety Inspection Program (OMC Title 8, Chapter 17; the city page cites inspections every four years and a $51 annual fee per unit). Ord. 3323 also deleted the 2025 text's annual rent-reporting duty for ADU owners.