The present invention relates to an arrangement system for shutters of an automatic floor door assembly within a lift plant such that the overall dimensions of the door sill can be reduced to the minimum. The shutter moving is realized by carriages, each of them fixed to each shutter of said door assembly, with a geometric configuration such that it is possible to displace on a single vertical plane sliding guides for said carriages and the moving members in a space limited by said vertical plane. The sill overall dimensions save allows for installing such automatic lift doors in lift shaft with clearance insufficient for conventional automatic doors.