California Code, Vehicle Code - VEH § 38603

(a) A person operating a recreational off-highway vehicle with a model year of 2014 or later on lands, as described in Section 38001, shall not allow a passenger to occupy a separate seat location not designed and provided by the manufacturer for a passenger.

(b) Seats that are installed in a separate seat location not designed and provided by the manufacturer for a passenger in a vehicle with a model year of 2013 or earlier on lands, as described in Section 38001, may be occupied by a passenger if the occupant of the seat is fully contained inside of the vehicle's rollover protection structure at all times while the vehicle is being operated.