…assuming that it does not have a separate service.
Maybe on a two-wire system with plastic conduit?
I guess another way of asking is: under what conditions would you want to see a panel for a separate structure (supplied by the house service) have its own GEC and electrode?
It looks to me:
If plastic conduit is used between the house and the separate structure:
If the panel in the separate structure has only one branch circuit, with no overcorrect protection, then that panel is treated as a typical distribution panel, the grounding an neutral bus bars are isolated from each other, and no GEC and electrode are needed.
If the panel in the separate structure has 2- 120V supply bus bars (multiple circuits), supplied by a 4-wire feeder (1 blk, 1 red, 1white, and one green), but has overcorrect protection installed then that panel is still treated as a distribution panel with the grounding and neutral bus bars isolated from each other, but the grounding bus bar should be connected to a GEC and electrode serving the separate panel.
The images are from the 2014 NEC NFPA Handbook. I think I’be interpreting this correctly, although I might not be describing it using accurate/the best terms.