Abstract: The invention relates to a telescopic staircase to be installed on construction sites for providing a safe staircase for workers during building construction. The telescopic staircase comprising at least one lateral guard comprising a top rail, a bottom rail and at least two newels, the bottom rail having a plurality of connecting holes, and a plurality of tread assembly comprising a tread plate, at least two stringer portions and at least two pairs of support members, each support member having at least one connecting hole, in which the at least one lateral guard includes a first portion being retractable into and extensible from a second portion, in which each support member substantially forms a 45° angle with the tread plate, and in which the support members are to be attached to the top surface of the bottom rail via attachment means through the connecting holes.