Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Ontario ADU cost →Building an ADU in Ontario costs between $180,000-$300,000 for new construction, with per-square-foot costs averaging $200-$275. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $80,000-$150,000, while prefab/modular ADUs fall in between at $80,000-$150,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | $3,000-$6,000 estimated (building permit + plan check); minimum $47.50 per state |
| Impact Fees | None on an ADU of 750 sq ft or less of interior livable space (Gov. Code 66311.5(c)(1)); charged proportionately above that; proportional for larger units |
| Site Prep & Foundation | $10,000 – $35,000 |
| Framing & Structure | $15,000 – $45,000 |
| Plumbing | $8,000 – $20,000 |
| Electrical | $5,000 – $15,000 |
| HVAC | $3,000 – $8,000 |
| Interior Finishes | $15,000 – $40,000 |
Quotes for the same ADU commonly vary 20–40% between builders. Get up to 5 free quotes from ADU builders serving Ontario — no obligation.
Get Ontario builder quotes →Permit fees in Ontario are $3,000-$6,000 estimated (building permit + plan check); minimum $47.50 per state. The review timeline is 30-60 days typical for compliant applications, with plan check: Typically 2-3 weeks for first review. Impact fees are None on an ADU of 750 sq ft or less of interior livable space (Gov. Code 66311.5(c)(1)); charged proportionately above that; proportional for larger units. Pre-approved plans: State of California offers pre-approved ADU plans.
| 1-Bedroom Rent | $1,400-$1,800/month |
| 2-Bedroom Rent | $1,800-$2,300/month |
| Annual Rental Income (est.) | $19,200/year |
| Payback Period (est.) | 12.5 years |
| Short-Term Rental | Subject to local restrictions; check city ordinance |
| Overall ROI Estimate | 8-12 year payback; 5-8% annual ROI |
A typical ADU in Ontario generates approximately $19,200 per year in rental income based on average 1-bedroom rents. At an average construction cost of $240,000, the estimated payback period is 12.5 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Ontario and varies by market (often less than build cost).
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in Ontario: $80,000-$150,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $80,000-$150,000.
3. Pre-approved plans — available in Ontario, which can save $3,000-$10,000 in design costs and 2-3 months in review time. See which cities offer pre-approved ADU plans.
4. State grants — California's CalHFA ADU grant (up to $40K) is currently paused; check calhfa.ca.gov/adu for status and your state for active programs.
5. Get multiple bids — ADU contractor pricing varies 20-40% for the same project. We recommend getting at least 3 quotes.
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