Can I Build an ADU in Ontario?

Yes — by right

ADUs and JADUs are allowed by right in all residential and mixed-use zones; ministerial approval, no hearing.

1 ADU + 1 JADU (one ADU per single-family lot)

Ontario allows a maximum of one ADU per single-family lot (lots with multiple detached single-family dwellings are still limited to one ADU total) plus one JADU. The JADU must be contained within the single-family dwelling or its attached garage.

Up to 2 detached ADUs + conversion ADUs (≥1, up to 25% of existing units)

On a multifamily lot Ontario allows two detached ADUs (which may be attached to each other) plus conversion of non-habitable space for at least one ADU or up to 25% of the existing units. Attached ADUs are not allowed on multifamily lots. Note state law (§ 66323) now requires cities to permit up to 8 detached ADUs on a qualifying multifamily lot, so this lower local cap is likely preempted — confirm with Planning.

Allowed

Up to 500 sq ft, within the walls of an existing single-family home (single-family lots only).

1,000 sq ft

Ontario limits a detached ADU to 850 sq ft (studio/1-bedroom) or 1,000 sq ft (2+ bedrooms) — it does NOT grant the larger 1,200 sq ft state default ceiling, so the effective detached max is 1,000 sq ft. The 800 sq ft state floor is still guaranteed (no lot-coverage/open-space rule may push an ADU below 800 sq ft).

850 sq ft (studio/1-BR) or 1,000 sq ft (2+ BR), and ≤50% of the primary dwelling's floor area; never required below 800 sq ft

Ontario caps an attached ADU at the 850/1,000 sq ft size minimums AND 50% of the existing primary dwelling's floor area, but no percentage/lot-coverage/open-space limit may require an ADU smaller than 800 sq ft (§ 5.03.010 fn 1).

800 sq ft

An 800 sq ft ADU must be allowed regardless of FAR/lot-coverage caps; a city also cannot cap an attached ADU below 850 sq ft (1,000 for 2+ bedrooms).

500 sq ft

16 ft

16 ft baseline; cities must allow 18 ft within ½ mi of a major transit stop or where the primary dwelling is two-story, and 20 ft on a lot with a multistory multifamily building.

4 ft

A city cannot require more than 4 ft side or rear setback; conversions of existing structures are exempt.

4 ft

No front setback may block the mandatory 800 sq ft / 16-ft / 4-ft-setback ADU; otherwise per base zone

6 ft

Ontario requires 6 ft of separation between detached structures. State law sets no separation minimum, so this is a genuine local standard.

Up to 1 space

At most 1 space per ADU (or per bedroom, whichever is less) — and waived in the common cases below.

Yes — no parking required near transit & for conversions

No parking may be required within ½ mi walking distance of transit, in a historic district, for an ADU within an existing structure, within one block of car-share, or in certain permit-parking areas.

Not required

When a garage/carport is demolished or converted to build an ADU, replacement parking cannot be required.

Ministerial (by right)

No public hearing or discretionary design review for a qualifying ADU.

60 days

The agency must act within 60 days of a complete application — and the ADU is deemed approved if it does not.

Only for a JADU

No owner-occupancy for a standalone ADU. For a JADU, the owner must occupy either the primary residence or the JADU.

Waived under 750 sq ft

An ADU of 750 sq ft or less pays no impact fees (the 750 sq ft threshold itself is exempt under § 66311.5); larger ADUs pay fees proportional to the primary dwelling. JADUs (≤ 500 sq ft) are exempt.

Building permit

Only a ministerial building permit — no conditional use permit or discretionary design review for a qualifying ADU.