A window installation system, comprising: a roof window including a window frame having a top frame member, a bottom frame member defining an inner width (wi) of the window frame, and opposing first and second side frame members, defining a central opening, each of the frame members (300, 200, 100) having an inner face including an interior portion (301, 201, 101) and an exterior portion (202, 102) facing the central opening, and a sash having a top sash member, a bottom sash member and opposing first and second side sash members, together encasing a pane and connected to the window frame in a mounted condition by means of a set of hinges, and a kit of parts for protecting the window frame in a window installation process bringing the roof window to the mounted condition, in which the kit of parts comprises at least one protection member adapted to be positioned on the bottom frame member to cover at least the interior portion of the bottom frame member, wherein the kit of parts comprises a bottom protection member having a pre-defined length (lb) and at least one side protection member (12, 13) having a pre-defined length (Is), said at least one side protection member (12, 13) being configured to be connected to the bottom protection member, thereby forming a protection unit; and the protection unit is adapted for covering at least partly the inner face of the bottom frame member and the inner face of at least one of the first and second side frame members; wherein the bottom protection member and the at least one side protection member are configured to be assembled, thereby forming the protection unit, characterised in that the bottom protection member (3011; 4011; 7011; 9011) is composed of a plurality of pieces (3111a, 3111b, 3111c, 3111d; 4111a, 4111b, 4111c, 4111d, 4111e; 7111a-7111x), preferably having an individually adapted profiling, and wherein at least one piece (4111a, 4111d; 9111c, 9111a) of the bottom protection member (4011; 9011) is integral with an associated side protection member (4013, 4012; 9013, 9012) to form a corner unit, and wherein the bottom protection member (4011; 9011) preferably comprises at least one other piece (4111b, 4111c, 4111e; 9111b) connected to or connectable to said at least one piece (4111a, 4111d; 9111c, 9111a) of the bottom protection member (4011; 9011) in the condition of use, and that the pieces (7111a-7111x) are connected to each other by a piece connection fitting allowing releasable snap connection between the pieces (7111a-711x) of the bottom protection member of the protection unit. a roof window including a window frame having a top frame member, a bottom frame member defining an inner width (wi) of the window frame, and opposing first and second side frame members, defining a central opening, each of the frame members (300, 200, 100) having an inner face including an interior portion (301, 201, 101) and an exterior portion (202, 102) facing the central opening, and a sash having a top sash member, a bottom sash member and opposing first and second side sash members, together encasing a pane and connected to the window frame in a mounted condition by means of a set of hinges, and a kit of parts for protecting the window frame in a window installation process bringing the roof window to the mounted condition, in which the kit of parts comprises at least one protection member adapted to be positioned on the bottom frame member to cover at least the interior portion of the bottom frame member, wherein the kit of parts comprises a bottom protection member having a pre-defined length (lb) and at least one side protection member (12, 13) having a pre-defined length (Is), said at least one side protection member (12, 13) being configured to be connected to the bottom protection member, thereby forming a protection unit; and the protection unit is adapted for covering at least partly the inner face of the bottom frame member and the inner face of at least one of the first and second side frame members; wherein the bottom protection member and the at least one side protection member are configured to be assembled, thereby forming the protection unit, characterised in that the bottom protection member (3011; 4011; 7011; 9011) is composed of a plurality of pieces (3111a, 3111b, 3111c, 3111d; 4111a, 4111b, 4111c, 4111d, 4111e; 7111a-7111x), preferably having an individually adapted profiling, and wherein at least one piece (4111a, 4111d; 9111c, 9111a) of the bottom protection member (4011; 9011) is integral with an associated side protection member (4013, 4012; 9013, 9012) to form a corner unit, and wherein the bottom protection member (4011; 9011) preferably comprises at least one other piece (4111b, 4111c, 4111e; 9111b) connected to or connectable to said at least one piece (4111a, 4111d; 9111c, 9111a) of the bottom protection member (4011; 9011) in the condition of use, and that the pieces (7111a-7111x) are connected to each other by a piece connection fitting allowing releasable snap connection between the pieces (7111a-711x) of the bottom protection member of the protection unit.