An extendable belt conveyor system designed to be installed under a self-propelled windrower between the front and the rear wheels suitable for multiple windrowing operations. The self-propelled windrower with the extendable belt conveyor system deliver a swath at a variable distance from the windrower, either on left or on the right, as the machine moves through the field. The conveyor can be directed on the left or on the right at variable distances with no need to manually change a part. The extendable belt conveyor system has at least two conveyors disposed one above the others and are connected with a trolley between each conveyor. The extendable belt conveyor system is driven by a single driving means and by a single reversible motor.