Ontario, CA Accessory Structures: ADU Rules (2026)
ADU rules in Ontario, CA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- 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
- Detached height
- 16 ft (18 ft near transit; 20 ft to match roof pitch)
- Attached height
- 25 ft or zone limit, max 2 stories
- Setbacks
- 4 ft rear, interior side, and street side
- Parking
- 1 space per ADU (waived per state exceptions)
- Owner occupancy
- Not required for ADUs
- Rental
- No short-term rental; 30-day minimum
- Review
- Ministerial, 60-day clock
Summary
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.
5.03.010: Accessory Dwelling Units. A. Purpose. The purpose of this Section is to allow and regulate accessory dwelling units (ADUs) and junior accessory dwelling units (JADUs) in compliance with Chapter 13 of Division 1 of Title 7 of the California Government Code. B. Effect of Conforming. An ADU or JADU that conforms to the standards in this Section will not be: 1. Deemed to be inconsistent with the City's general plan and zoning designation for the lot on which the ADU or JADU is located. 2. Deemed to exceed the allowable density for the lot on which the ADU or JADU is located. 3. Considered in the application of any local ordinance, policy, or program to limit residential growth. C.1. Accessory Dwelling Unit (ADU). An attached or a detached residential dwelling unit that provides complete independent living facilities for one or more persons and is located on a lot with a proposed or existing primary residence.
Full Breakdown
010 of the Ontario Development Code allows and regulates accessory dwelling units (ADUs) and junior accessory dwelling units (JADUs) in compliance with Chapter 13 of Division 1 of Title 7 of the California Government Code (Gov. ). An ADU that conforms to the Section is not deemed inconsistent with the City's general plan or zoning, does not exceed allowable density, and is not considered when applying any local growth-limit ordinance. ADUs and JADUs are processed ministerially; an application is approved or denied within 60 days.
Detached and attached ADUs are limited to 850 square feet for a studio or one-bedroom unit and 1,000 square feet for two or more bedrooms, with attached ADUs further limited to 50 percent of the floor area of the existing primary dwelling. No size, lot-coverage, or open-space limit may require an ADU to be less than 800 square feet. A detached ADU may not exceed 16 feet in height, except up to 18 feet within one-half mile of a major transit stop or high-quality transit corridor (20 feet to match the primary roof pitch), and up to 18 feet on a lot with a multi-story multi-family dwelling; an attached ADU may not exceed 25 feet or the underlying zone height limit, whichever is lower, and cannot exceed two stories.
New ADUs require minimum 4-foot rear, interior side, and street side setbacks, with front-yard setbacks following the underlying zoning district; a 6-foot separation is required between detached structures. b. Owner-occupancy is not required for ADUs (it applies only to JADUs). ADUs may not be used as short-term rentals and may be rented only for terms of 30 days or more. Impact fees apply only to units of 750 square feet or larger.
Violations & Fines
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.
Frequently Asked Questions
How large can an ADU be in Ontario, CA?
What setbacks apply to an ADU in Ontario?
Does Ontario require the owner to live on-site with an ADU?
Can an Ontario ADU be a short-term rental?
Sources & Official References
Other rules in Ontario
How Ontario compares: Easiest Cities to Build an ADU·California rules heatmap·Compare Ontario to another location·View the California accessory structures overview
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ADU Rules in Nearby Cities
How other cities in San Bernardino County handle adu rules.