Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Building an ADU in Fontana costs between $160,000-$270,000 for new construction, with per-square-foot costs averaging $200-$250. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $75,000-$130,000, while prefab/modular ADUs fall in between at $80,000-$140,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | Waived for ADUs under 750 sq ft; fee applies for larger units (amount varies) |
| Impact Fees | Waived for units under 750 sq ft (SB 13); 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 |
Permit fees in Fontana are Waived for ADUs under 750 sq ft; fee applies for larger units (amount varies). The review timeline is ~60 days with pre-approved plans, with plan check taking approximately Included in 60-day timeline; faster with pre-approved designs. Impact fees are Waived for units under 750 sq ft (SB 13); proportional for larger units. Pre-approved plans: AB 1332 standard ADU plans available on City website.
| 1-Bedroom Rent | $1,300-$1,700/month |
| 2-Bedroom Rent | $1,700-$2,200/month |
| Annual Rental Income (est.) | $18,000/year |
| Payback Period (est.) | 11.9 years |
| Short-Term Rental | Subject to local ordinance; verify before planning |
| Overall ROI Estimate | 7-10 year payback |
A typical ADU in Fontana generates approximately $18,000 per year in rental income based on average 1-bedroom rents. At an average construction cost of $215,000, the estimated payback period is 11.9 years — before accounting for property value appreciation, which typically adds 20-35% to home value.
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in Fontana: $75,000-$130,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $80,000-$140,000.
3. Pre-approved plans (AB 1332 standard ADU plans available on City website) can save $3,000-$10,000 in design costs and 2-3 months in review time.
4. State grants — California's CalHFA program offers up to $40K. Check your state for available 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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