Abstract: The disclosure relates to a modular side post system (1) for a roll up door (2) comprising a first side post part (3,33) comprising a profile body (5,35) comprising a fastening surface (11) arranged to fasten around a door hole of a wall and a slit (12) arranged to guide a door curtain of the roll up door (2), a first fitting geometry (6,36) at a first peripheral end of the profile body (5,35), and a second fitting geometry (7,37) at second peripheral end of the profile body (5,35); at least one second side post part (4,34), wherein the second side post part is identical to the side post part (3,33), wherein the first fitting geometry (6,36) of the first side post part is adapted to fit the second fitting geometry (7,37) of the second side post part (4,34) so that the slit (12) of the first side post part (3,33) continues uninterrupted to the second side post part (4,34). The disclosure further relates to a method for arranging a modular side post system (1) for a roll up door (2) at a door hole in a wall.