Around here for years when using a trench for burying USE cable in residential areas, it's always said that the requirement was 18", but as I read in the code it's actually 24" unless it's under a residential driveway or parking area. I'm talking about a normal run to a out building from a main panel not running under any concrete or not in conduit. Am I the only one that's been with this assumption? I've think I've even had inspectors tell me 18" before. Any thoughts on this??? Thanks...
Also when driving ground rods for a subpanel in a detached building, I'm under the assumption that it is still (2) ground rods at least 6ft. apart and attached to the grounding bar in the subpanel...???

300.5 says 24" for direct burial cable/conductors unless this is a residential, single circuit, GFCI at the source end, 120v, 20a max, which can be 12"
That loophole also eliminates the need for a ground rod.

Anything else puts you into 225 part II and 250.32 along with the deeper 300.5 requirements.

They put the single residential circuit loophole in there to compromise a safer alternative to running an orange cord through the trees or along the ground.
If digging is hard there, RNC at 18" becomes a very attractive option. Personally I would go that way anyhow.

The 6' rule is valid but i don't think you need two ground rods unless the resistance of the one electrode is higher than 25 ohms.