Can I power an ADU or second structure from my existing house panel?

Often yes, through a feeder to a subpanel in the second building, if the house panel has enough capacity. The NEC requires a disconnecting means at the detached building, an equipment grounding conductor run with the feeder, and the neutral and ground kept separate in the outbuilding panel, according to Minnesota's electrical inspection bulletin. The NEC generally allows only one feeder or service to a building, with listed exceptions. Capacity is the common obstacle: a national lab study found at least 20% of panels have no spare breaker space. An electrician should run a load calculation first; some homes need a panel upgrade or load-management devices before adding a feeder.

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
• Characterizing Electrical Panel Capacity, Breaker Space, and Loads in U.S. Single-Family HomesLawrence Berkeley National Laboratory
• Home Electrification and Electric Panel UpgradesU.S. Department of Energy, Building Science Education Solution Center