An access door adjustable to an opening, comprising a frame adjustable to the width and the height of the opening; and a door panel, the frame comprising side members and corner members; each side member having a section adapted to receive an edge of the opening and comprising a first part adapted to lay flat on an outer surface of the edge of the opening and a second part extending within the opening and comprising engagement positions, the engagement positions being adapted to engage the corner members between a maximum length for each side of the frame by using outermost engagement positions and a minimum length for each side of the frame, depending on the size of the opening, the frame receiving the door panel for closing the opening.