Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Fontana ADU cost →Building an ADU in Fontana costs between $175,000-$320,000 for new construction, with per-square-foot costs averaging $180-$300. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $75,000-$150,000, while prefab/modular ADUs fall in between at $120,000-$250,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | No flat ADU fee: building permit and plan check are valuation-based per the Comprehensive Fee Schedule (Res. 2024-054, 9/1/2024), plus Planning plan check $191, Engineering $120 and Fire $90 per submittal and small state/archive add-ons. Get a quote from Building & Safety, (909) 350-7640. |
| Impact Fees | None on an ADU of 750 sq ft or less (Gov. Code 66311.5(c)(1)). Above 750 sq ft, the city's per-unit single-family development fees (about $20,600 in Planning fees plus $8,620 sewer expansion and $1,023 sewer connection per unit on the 10/2024 sheet) are charged in proportion to the ADU's share of the main house's floor area. IEUA sewer and school-district fees are billed separately. |
| 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. We pass your request to up to 5 local builders serving Fontana, where we have them — free, no obligation.
Get Fontana builder quotes →Permit fees in Fontana are No flat ADU fee: building permit and plan check are valuation-based per the Comprehensive Fee Schedule (Res. 2024-054, 9/1/2024), plus Planning plan check $191, Engineering $120 and Fire $90 per submittal and small state/archive add-ons. Get a quote from Building & Safety, (909) 350-7640.. The review timeline is Building & Safety's published plan-check times: first review about 3-4 weeks, each recheck about 2 weeks. State law requires a decision within 60 days of a complete application and deems it approved if the city misses the deadline (Gov. Code 66317, SB 543, Jan. 2026)., with plan check: First review about 3-4 weeks; rechecks about 2 weeks after corrected plans are resubmitted; over-the-counter issuance only for work that needs no plan check. Status is tracked in the Build Fontana portal or at (909) 350-7640. Fire District reviews (NFPA 13D sprinklers) run separately with a 15-working-day goal.. Impact fees are None on an ADU of 750 sq ft or less (Gov. Code 66311.5(c)(1)). Above 750 sq ft, the city's per-unit single-family development fees (about $20,600 in Planning fees plus $8,620 sewer expansion and $1,023 sewer connection per unit on the 10/2024 sheet) are charged in proportion to the ADU's share of the main house's floor area. IEUA sewer and school-district fees are billed separately.. Pre-approved plans: Yes. Fontana's Standard ADU Plans program (AB 1332, Gov. Code 65852.27) posts three pre-approved detached plans: 462 sq ft (1 bed/1 bath), 738 sq ft (2 bed/2 bath) and 1,143 sq ft (3 bed/2 bath). The plan sets are free to download from the city site; you still submit a site-specific plan and pay permit fees..
| 1-Bedroom Rent | $1,750-$1,900/month |
| 2-Bedroom Rent | $2,100-$2,300/month |
| Annual Rental Income (est.) | $21,900/year |
| Payback Period (est.) | 11.3 years |
| Short-Term Rental | A JADU may not be rented for less than 30 days (Gov. Code 66333, AB 1154). The city's minimum rental term for ADUs sits in Sec. |
| Overall ROI Estimate | 7-15% gross rental yield on a $175,000-$320,000 detached build at $1,750-$2,300/mo (about a 7-15 year simple payback); roughly 14-35% gross on a $75,000-$150,000 garage conversion; before taxes, vacancy and financing |
A typical ADU in Fontana generates approximately $21,900 per year in rental income based on average 1-bedroom rents. At an average construction cost of $247,500, the estimated payback period is 11.3 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Fontana 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 Fontana: $75,000-$150,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $120,000-$250,000.
3. Pre-approved plans — available in Fontana, 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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