Ontario's Accessory Structures: The Rules That Matter

Every city handles accessory structures a little differently. In Ontario, California, there are 5 distinct rules that residents and property owners should be aware of. Some are stricter than what neighboring cities enforce, and others are more relaxed. Here is what you need to know.

Tiny Homes

Foundation-built tiny homes qualify as ADUs under Gov Code 65852.2 in Ontario. Tiny homes on wheels are treated as RVs or park trailers and cannot be used as permanent dwellings on residential lots.

Key details: Foundation tiny home: Legal as ADU under Gov 65852.2. THOW (wheels): Treated as RV or park trailer. Permanent THOW living: Not allowed on residential lot. ADU size cap: Up to ~1,200 sq ft per state. Permit: Ontario Building and Safety.

Living in a tiny home on wheels as a primary residence on a residential lot is routinely cited by Code Enforcement. Unpermitted foundation-built tiny homes can be cited as unpermitted structures and required to be permitted or removed.

This is not one of those rules that cities tend to ignore. Ontario actively enforces its tiny homes requirements.

Carport Rules

Ontario allows carports on residential lots subject to zoning setbacks and a building permit for any carport 200 sq ft or larger. Properly sized carports count toward required covered parking.

Key details: Permit threshold: 200 sq ft or any permanent. Setbacks: Usually 3-5 ft rear/side. Counts as parking: If 9 ft x 18 ft minimum. Wind load: CBC Risk Category II. HOA note: Fabric carports often banned.

Installing a permit-required carport without a permit can lead to Code Enforcement citations, stop work orders, and a requirement to obtain retroactive permits or remove the structure. Non-compliant fabric carports are frequently cited in the front yard by HOAs.

Garage Conversions

Ontario allows converting an existing garage or accessory structure into an ADU under Section 5.03.010, waiving replacement parking for the converted or demolished garage, requiring a 3-foot landscape planter where the garage door is removed, and allowing the existing structure's location and setbacks to be retained.

Key details: Code Section: Ontario Development Code 5.03.010. State law: Cal. Gov. Code 66323 (Class 1 ADUs). Replacement parking: Not required when garage demolished/converted for ADU. Existing setback: No setback required for converted existing structure. Added space: Up to 150 sq ft for ingress/egress.

Converting a garage to a dwelling unit without the required ministerial building permit, or failing to provide the required landscape planter where the garage door is removed, is a Development Code violation subject to code-enforcement action, including correction and after-the-fact permitting. A conversion that does not conform to the objective standards of Section 5.03.010 may be allowed only through a discretionary Conditional Use Permit.

ADU Rules

Ontario permits accessory dwelling units ministerially under Section 5.03.010 of the Development Code, limiting detached and attached ADUs to 850 square feet for a studio/one-bedroom and 1,000 square feet for two or more bedrooms, with 4-foot rear and side setbacks, a 16-foot detached height limit, and one parking space per unit.

Key details: Code Section: Ontario Development Code 5.03.010. State law: Cal. Gov. Code 66310 et seq. (Title 7, Div. 1, Ch. 13). Max ADU size: 850 sq ft (studio/1-BR); 1,000 sq ft (2+ BR). Attached ADU cap: 50% of primary dwelling floor area. Floor-area floor: May not be forced below 800 sq ft.

An ADU or JADU that does not conform to the objective standards of Section 5.03.010 may be allowed only through a discretionary Conditional Use Permit; a CUP is also required for an ADU exceeding the allowable height, up to a maximum of 35 feet. Constructing an ADU without the required ministerial building permit, or in violation of the Development Code, is subject to code-enforcement action, including correction and after-the-fact permitting, and the City may enforce compliance with applicable building standards under Health and Safety Code Section 17980.12.

Ontario is more permissive than most cities when it comes to adu rules. That said, there are still limits.

Shed Rules

Ontario Development Code Section 5.03.011 governs detached accessory residential structures such as storage sheds, capping permitted-by-right area at 650 square feet (up to 1,100 for required garage parking) and 16 feet in height, requiring 5-foot rear and 10-foot interior side setbacks, prohibiting kitchens, and barring placement in front yards or shipping containers as storage.

Key details: Code Section: Ontario Development Code 5.03.011; Table 5.03-1. Max height (by right): 16 ft (35 ft with CUP). Max area (by right): 650 sq ft (1,100 sq ft for required garage parking). Rear setback: 5 ft (under 25 ft wide); 10 ft (25 ft+ wide). Street side setback: 10 ft.

Building or maintaining a detached accessory structure that exceeds the permitted-by-right height (16 feet) or area (650 square feet) without a Conditional Use Permit, that encroaches on the Table 5.03-1 setbacks, that is placed in a front yard, or that includes a prohibited kitchen or shipping container, is a Development Code violation subject to code-enforcement action, including abatement, after-the-fact permitting, and potential CUP requirements. The Planning Director may require a recorded restrictive covenant before issuing a building permit for an accessory residential structure.

The Bottom Line

Ontario's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Ontario is broadly strict or permissive.

Keep in mind that Ontario can amend these rules at any council meeting. For the most current version of any rule mentioned here, check the specific ordinance page, where we track updates as they happen.