As of October 2026, California guarantees room for an ADU of at least 800 sq ft with 4-foot side and rear setbacks, and a city's size cap cannot fall below 850 sq ft, or 1,000 sq ft with more than one bedroom. Without a compliant local rule, a detached ADU can reach 1,200 sq ft. Junior ADUs top out at 500 sq ft.
What size limits does state law set?
| Rule | Size | Code |
|---|---|---|
| Guaranteed ADU with 4-ft setbacks | 800 sq ft | §66321(b)(3) |
| Lowest allowed local cap | 850 sq ft; 1,000 sq ft with more than one bedroom | §66321(b)(2) |
| Detached cap without a compliant ordinance | 1,200 sq ft | §66314(d)(4) |
| Attached cap | 50% of the main home, but not below the floors above | §66314(d)(4) |
| Junior ADU | 500 sq ft | §66333 |
How tall can an ADU be?
A detached ADU can be 16 feet tall, or 18 feet within a half-mile of a major transit stop or high-quality transit corridor, plus up to 2 feet to match the main roof. An attached ADU can reach 25 feet or the zone's limit, whichever is lower.
How did Modesto's rules change?
In November 2023, HCD found that Modesto's ADU ordinance capped attached ADUs at 50 percent of the main home without protecting the 800 sq ft minimum, among other issues. The City Council adopted a zoning code amendment in late 2024 to match state law, according to the city.
We check the current Modesto code for your parcel before sizing a design.
How much can a garage conversion grow?
Only up to 150 sq ft beyond the existing structure, and only for entry and exit, under Government Code §66323(a)(1)(A).
Does a bigger ADU cost more in fees?
Yes, past 750 sq ft. Below that size, an ADU pays no impact fees; above it, fees scale with the unit's square footage, as of October 2026.
When is the biggest ADU not the best choice?
It depends on your goal and your yard. A 1,192 sq ft unit gives a family room to live, but it pays impact fees and takes more of the lot. A 744 sq ft unit avoids impact fees and leaves more yard. We size the design to the use and the budget.
Related
Sources, as of October 2026: HCD ADU Handbook (March 2026); California Government Code §§66311.5, 66314, 66321, 66323 and 66333; HCD letter to the City of Modesto (November 27, 2023); City of Modesto news release (March 7, 2025).
Updated October 2026