Cal. Veh. Code § 35402

(a) Any extension or device, including any adjustable axle added to the front or rear of a vehicle, used to increase the carrying capacity of a vehicle shall be included in measuring the length of a vehicle, except that a drawbar shall not be included in measuring the length of a vehicle but shall be included in measuring the overall length of a combination of vehicles.
(b) Notwithstanding subdivision (a), extensions of not more than 18 inches in length on each end of a vehicle or combination of vehicles used exclusively to transport vehicles shall not be included in measuring the length of a vehicle or combination of vehicles when the vehicles are loaded.
(c) Notwithstanding subdivision (a), an extension of not more than 18 inches in length on the last trailer in a combination of vehicles transporting loads shall not be included in measuring the length of a vehicle or combination of vehicles when the vehicles are loaded. Additionally, an extension of not more than 18 inches in length on the front of the first trailer in a combination of vehicles transporting loads shall not be included in measuring the length of a vehicle or combination of vehicles when the vehicles are loaded and on highways, other than those highways designated by the United States Department of Transportation as national network routes.
(d) Notwithstanding subdivision (a), any extension or device which is not used to carry any load and which does not exceed three feet in length, added to the rear of a vehicle, and is used exclusively for pushing the vehicle or a combination of vehicles, which vehicle or combination of vehicles is designed and used exclusively to transport earth, sand, gravel, and similar materials, shall be included in measuring the length of the vehicle but shall not be included in measuring the overall length of the combination of vehicles.
(e) Notwithstanding subdivision (a), a truck semitrailer combination, but not a truck tractor and semitrailer combination, may use a sliding fifth wheel, or a truck tractor, semitrailer, trailer, and a truck-trailer combination may use a sliding drawbar, to extend the length of the combination by not more than 2 feet 6 inches while traveling 35 miles per hour or less on any highway, except a freeway. These provisions shall apply, however, to freeway onramps and offramps and freeway connectors. The sliding fifth wheel or drawbar when extended shall not be included in measuring the overall length of the combination of vehicles if the pivot point of the semitrailer connection is more than two feet to the rear of the center of the rearmost axle of the motortruck or if the distance from the pivot point to the center of the rearmost axle of the semitrailer does not exceed 34 feet.
Combinations of vehicles permitted by this subdivision shall be in compliance with the weight limits provided in Article 1 (commencing with Section 35550) of Chapter 5 whenever any drawbar or sliding fifth wheel is extended, contracted, or in any intermediate position as provided for by this subdivision.