California bans certain features on Semi-automatic Shotguns.
12276.1. (a) Notwithstanding Section 12276, "assault weapon" shall also mean any of the following:
(6) A semiautomatic shotgun that has both of the following: (A) A folding or telescoping stock. (B) A pistol grip that protrudes conspicuously beneath the action of the weapon, thumbhole stock, or vertical handgrip. (7) A semiautomatic shotgun that has the ability to accept a detachable magazine. (8) Any shotgun with a revolving cylinder.
Note that you may have a folding stock as long as you do not also have a pistol grip or thumbhole stock and vise-versa.
Only both of these features combined trigger "assault weapon" status.