A preformed formwork for forming concrete floor slab for a height adjustable shipping container building structure. The preformed formwork (100) includes a frame having spaced apart side beams (102, 104), spaced apart end beams (101, 103), and fork lifting supports (106) that equally spaced apart from the centre of the frame extending between the spaced apart side beams (102, 104). Reinforcing (107) is situated within the internal area of the frame and attached to internal surface of the side and end beams (101, 102, 103, 104). Wall receiving feature on the external surface of the side and end beams to accommodate, support and attach a portion of wall panel to the end and side beams. Columns (105) are engaged and supported at the corners of the frame. The preformed formwork is dimensioned to equal the floor area of a standard shipping container such that the preformed formwork is able to be readily and easily transported within a height adjustable shipping container containing building components and able to be readily and easily erected and assembled as ether a ground floor or upper level floor of a building structure prior to been formed in to a concrete slab.