A motor vehicle seat assembly has a longitudinally movable rear seat and a load floor, which is height-adjustable, can be placed in a boot and has a front portion coupled to a lever member in a rotational manner about a horizontal axis; the lever member can swing about a further horizontal axis, which is fixed with respect to the rear seat during the longitudinal movement of the rear seat; the load floor has a substantially flat rear panel, which is coupled in a sliding manner to the front portion so as to vary the length of the load floor depending on the longitudinal position of the rear seat.