As of October 2026, we found no reputable Modesto-only price survey. The closest published benchmark, Block Renovation's June 2026 Sacramento guide, puts junior ADUs at $50,000 to $100,000, garage conversions at $80,000 to $150,000 and detached ADUs at $150,000 to over $300,000. Your site, utilities and size decide where you land.
What are the published ranges?
These are the nearest Central Valley figures we could source, as of October 2026.
| Build type | Range | Typical timeline |
|---|---|---|
| Junior ADU | $50,000 to $100,000 | 3 to 5 months |
| Garage conversion | $80,000 to $150,000 | 4 to 7 months |
| Attached ADU | $100,000 to $300,000 | 6 to 9 months |
| Detached ADU | $150,000 to $300,000+ | 6 to 10 months |
The same guide prices a 600 sq ft detached ADU with mid-range finishes at $180,000 to $260,000. Ranges are not quotes.
Do the free regional plans lower the cost?
They lower design cost. Modesto and its neighbors share free pre-drawn detached ADU plans, so the floor plan, structural and energy drawings are already done.
You still pay for a plot plan, trusses, solar, permits and construction. See our free plans guide.
Which line items move the budget most?
- Foundation and framing. Block's 2026 guide puts these at $50,000 to $80,000 for a 600 sq ft detached unit.
- Mechanical, electrical and plumbing. About $15,000 to $30,000 in the same example.
- Kitchen and bathroom. About $13,000 to $28,000.
- Solar. About $8,000 to $12,000 for a new detached unit.
- Utility connections. About $5,000 to $15,000, more for long runs.
- Electrical service. An older panel may need replacing before a second unit can be added.
Which fees does state law remove?
As of October 2026, Government Code §66311.5 exempts an ADU of 750 sq ft or less, and a junior ADU of 500 sq ft or less, from impact fees, and school fees do not apply below 500 sq ft.
Above those sizes, fees scale with square footage. Converted space and junior ADUs are also not treated as a new residential use for utility connection charges, unless built with a new home.
How much should I set aside for surprises?
Block Renovation's 2026 Sacramento guide recommends a 10 to 20 percent contingency, and notes that utility runs or poor soil can swing a total by $10,000 to $40,000.
Can I finance the build?
Modesto announced an ADU revolving loan fund with Valley First Credit Union in March 2025, with loans of up to $50,000 for eligible owners in city limits.
See our loan guide for the rules and what we could not confirm.
When does your lot decide the price?
It depends on hidden conditions. A long sewer run, an undersized panel or a slab that needs repair can separate two identical plans by a wide margin. We visit, measure and check the utilities, then give you a written, itemized quote rather than a guess.
Related
Sources, as of October 2026: Block Renovation, "Sacramento ADU Costs 2026: Types, Value, and Budget" (June 29, 2026); Stanislaus County ADU page; City of Modesto news release (March 7, 2025); California Government Code §66311.5; HCD ADU Handbook (March 2026). Published estimates, not our quotes.
Updated October 2026