ADU Placement Rules: What Actually Varies by Location

Accessory dwelling unit (ADU) rules are set at the state and local level, and they vary enormously — not just state to state, but often city to city within the same state.¹ There is no single national ADU rule. This guide walks through real, sourced examples from a few different jurisdictions to show you the range of what's possible, and what specifically to verify for your own property. It is not a substitute for calling your local planning department.

1. California: The Most Codified State-Level ADU Law

California has the most extensively legislated ADU framework in the country, and it keeps expanding. As of January 1, 2026, state law (Government Code §§66310–66342) sets specific, measurable standards local agencies cannot override:^2,3

• Setbacks: new construction detached ADUs generally require only 4 feet from side and rear property lines — a local agency cannot require more if doing so would prevent an 800 sq ft ADU from fitting.^2,3
• Height: up to 16 feet in most cases, up to 18 feet in specific circumstances, and up to 25 feet if attached to the primary dwelling.⁴
• Minimum size: typically around 150 square feet plus a code-compliant bathroom and closet; local agencies cannot set a minimum that blocks an efficiency unit.⁴
• No deed restrictions: a local agency cannot impose a deed restriction on an ADU.¹

Even within California, cities layer on their own specifics — San Diego, for example, allows combining multiple ADU types on one lot under a 2026 interpretation of state law, and caps most residential zones at 24–30 feet in height with some areas up to 35 feet.⁵

2. Texas: Local Caps Within a Lighter State Framework

Texas doesn't have the same statewide override framework as California; ADU rules are set more directly at the city level. In Dallas specifically, as of 2026: detached ADUs are capped at 800 square feet, with a 15-foot rear setback requirement and parking requirements that vary by unit size and location.⁶ Dallas is described as "moderately ADU-friendly" by one specialized ADU research source, with straightforward permitting but real attention needed to lot coverage limits.⁶

3. North Carolina: Size Tied to the Primary Home

North Carolina illustrates a third approach: sizing the ADU relative to the primary structure rather than a flat cap. In residential districts, an ADU generally can't exceed 50% of the primary structure's square footage, with specific setback rules that shift depending on whether the ADU is under or over 600 square feet.⁷ Notably, at least one North Carolina county's ordinance we reviewed explicitly prohibits manufactured homes from being used as an ADU⁷ — a restriction that would directly block certain prefab tiny home models regardless of size compliance, which is exactly the kind of detail a generic size chart won't surface.

What This Means for a Tiny or Container Home Specifically

ADU Placement Verification Checklist

1. Confirm ADUs are permitted at all on your specific zoning designation.
2. Ask explicitly whether manufactured, modular, or mobile structures qualify, or whether only site-built construction counts as an ADU locally.
3. Get the exact size cap and setback requirements in writing for your specific lot.
4. Check height limits against any loft or second-story element in your planned home design.
5. Ask about parking requirements, which can add unexpected site-prep cost.
6. If in California, confirm your city's implementation isn't imposing standards state law prohibits — this happens more often than it should.