An ADU cannot stand alone; it must always accompany a main house. California law and most city codes define an ADU as a secondary residential unit on the same lot as an existing or planned single-family or multifamily home. Since the ADU is accessory, it cannot exist as the only structure on a lot.
If your lot is vacant, you have two options. You can build a primary home and ADU together, which some cities allow by submitting plans at the same time. In this case, the city approves the ADU as accessory to the new home. Or, you can build the primary home first and add the ADU later once the main house is permitted or complete.