ADUs in Ontario, CA
Backyard homes for Ontario, priced before you sign.
We design, permit and build standard ADU models in Ontario and across the Inland Empire. You pick a model, we assess your lot, and you get one all-in price before you sign.
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The models
Four standard homes, each with a price up front.
Starting prices include design, permitting and construction with standard finishes. Site work and utilities generally run $15,000 to $35,000 and are priced after we see your lot, and city permit fees are billed at cost. All four fit the City's size caps for their bedroom count. See what's included.
Around the Inland Empire
Buildoly projects in the western Inland Empire.
Each pin is a Buildoly project between Ontario and Riverside. See the full map.
Ontario ADU rules in plain English
What you can build in the City of Ontario.
The basics for a single-family lot in Ontario, under the ADU rules the City adopted in December 2025. Every lot is different, so we confirm each of these for your address before we price anything.
How many
Up to two ADUs and a junior ADU
Under the state-law path in the City's code: one ADU inside the house or an existing accessory building, one junior ADU, and one new detached ADU of up to 800 sq ft. The City also allows its own ADUs of 850 to 1,000 sq ft, and we confirm with Planning how one fits with the others on your lot.
Ontario Dev. Code 5.03.010.D.3.aSize
850 or 1,000 sq ft
A City-standard ADU can be 850 sq ft as a studio or one bedroom, or 1,000 sq ft with two or more bedrooms. An attached one is also held to half the house, but never below 800 sq ft. The state-law detached ADU is capped at 800 sq ft.
Ontario Dev. Code 5.03.010.F.1; D.3.a(2)(b)Height
16 ft for a detached ADU
Every detached ADU can be 16 ft tall, or 18 ft within a half-mile walk of major transit, and up to 20 ft there to match the house's roof. A City-standard detached ADU is one story. An attached ADU can be 25 ft or the zone's limit, whichever is lower, and two stories at most.
Ontario Dev. Code 5.03.010.E.2; F.10Setbacks
4 ft from the side and rear
For a City-standard ADU, the front setback is 30 ft from an arterial street or 20 ft from a collector or local street, and it can't push the ADU under 800 sq ft. An ADU rebuilt in the same place and size as an existing structure needs no setback.
Ontario Dev. Code 5.03.010.F.2; D.3.a(2)(a)Parking
One space, with exceptions
A City-standard ADU needs one space per ADU or per bedroom, whichever is less, unless it's within a half-mile walk of transit, within a block of a car-share stop, in a historic district, part of the house or an accessory building, or where street permits aren't offered. State-law ADUs need none, and lost garage spaces don't have to be replaced.
Ontario Dev. Code 5.03.010.F.6; C.11Owning and renting
Rent for 30 days or more
You don't have to live on the property to have an ADU. ADUs and junior ADUs can't be rented for less than 30 days, or sold apart from the house except as state law allows.
Ontario Dev. Code 5.03.010.E.4, E.5, E.7Worth knowing in Ontario
A junior ADU needs the owner living in the house or the junior ADU, unless the junior ADU has its own bathroom. Every junior ADU also needs a deed restriction recorded before its certificate of occupancy.
The City's current ADU rules were adopted as an urgency ordinance in December 2025 and took effect right away.
Changing on January 1, 2027
SB 1117 limits the impact fee on an ADU over 750 sq ft to the area above 750 sq ft, in proportion to the main house, for up to two ADUs on a lot. ADUs of 750 sq ft or less still pay no impact fee.
AB 956 lets you build two new detached ADUs on a single-family lot through the state's approval path. On that path a city can cap each one at 800 sq ft.
Sources: Ontario Development Code Section 5.03.010, as restated by Urgency Ordinance No. 3323, adopted and in effect December 16, 2025; California Government Code sections 66310 to 66342; SB 1117 and AB 956 (2026). Checked November 2026.
How it works
From your lot to move-in.
Step 1
Pick a model
Choose a model and finishes online, or book a free consultation to talk it through.
Step 2
Site assessment
We visit your lot, check utilities, access and the City's rules, and give you one all-in price before you sign.
Step 3
Design and permits
We finish the plans and run the permit with the City of Ontario from submittal to approval.
Step 4
Build and move in
We build on site and manage the job through final inspection and your certificate of occupancy.
Typical timeline: 8 to 12 months from start to finish. About 1 month of design, 2 to 4 months of permits and 3 to 5 months of construction. Your lot and the City's review times can move it.
Ontario ADU questions
What Ontario homeowners ask us.
Our models start at $165,000 for Model S, $195,000 for Model 1, $270,000 for Model 2, $360,000 for Model 3. Those are all-in starting prices for design, permitting and construction. Site work and utilities generally run $15,000 to $35,000, and we price yours after the site assessment, before you sign. City permit fees are separate: your city sets them and we bill them at cost. You start with $1,000, credited in full to your contract, and many of our clients finance the whole build through our lending partners, subject to credit approval. See what's included in the price.
A City-standard ADU can be up to 850 sq ft as a studio or one bedroom, or 1,000 sq ft with two or more bedrooms, and an attached one is also held to half the house, but never below 800 sq ft. A detached ADU built under the state-law path is capped at 800 sq ft. All four of our models fit the City's caps for their bedroom count. We check the exact limit for your lot.
Our typical timeline is 8 to 12 months from start to finish: about 1 month of design, 2 to 4 months of permits and 3 to 5 months of construction. Your lot and the City's review times can move it either way, and we give you a schedule for your address after the site assessment.
A City-standard ADU needs one off-street space per ADU or per bedroom, whichever is less, unless it's within a half-mile walk of public transit (a bus stop counts), within one block of a car-share stop, in a historic district, part of the house or an accessory building, or where on-street permits are required but not offered to the occupant. ADUs built under the state-law path need none, and spaces lost to a garage conversion or demolition don't have to be replaced.
The City charges no impact fees on a junior ADU or on an ADU under 750 sq ft of interior living space. A larger ADU pays them in proportion to its size compared with the house. On a single-family lot with an existing house, a junior ADU or an ADU converted from existing space needs no new utility connection and pays no connection fee or capacity charge.
Yes. We build across the Inland Empire, as well as Los Angeles County, Orange County and San Diego. Each city has its own ADU rules and plan check, and we confirm them for your address. Enter your address in our quote tool to check that you're in our service area.
Where we build
We also build in these cities.
Each city page covers that city's ADU rules and who issues the permit. All cities.
Los Angeles County
Orange County
San Diego County
Inland Empire
See your price for an Ontario ADU.
Pick a model and tell us your address. After the site assessment you get one all-in number, before you sign.