Does a detached building like an ADU need its own ground rod?

Yes, under the NEC a separate building fed by a feeder generally needs its own grounding electrode system, in addition to the grounding conductor run from the main panel. The feeder's equipment grounding conductor is connected to that building's grounding electrode system at its disconnect, and the neutral and ground bars are kept separate there so ground wires don't carry normal current. A common electrode setup is two ground rods spaced at least 6 feet apart and joined by at least a #6 copper conductor, as the City of Austin's inspection checklist lists. Your inspector may require a concrete-encased electrode if the building has a new foundation.

Check your local rule. This varies by state, county or city. Your permitting office has the final word; the official lookup pages below show you who that is.

Sources

• Electrical Bulletin: Multiple Feeders / Feeders to Separate BuildingsMinnesota Department of Labor and Industry
• Residential Electrical Ground Inspection ChecklistCity of Austin Development Services