A lightweight carbon fiber reinforced polymer (CFRP) sandwich panel has been developed for floor use in commercial office building construction. CFRP laminate skins were combined with low-density rigid polyurethane foam to create a composite sandwich panel suitable for floor use. The CFRP sandwich panel was optimized to withstand code prescribed office-building live loads using a 3D finite element computer program called SolidWorks. The thickness of the polyurethane foam was optimized to meet both strength and serviceability requirements for gravity loading. Deflection ultimately was the controlling factor in the design, as the stresses in the composite materials remained relatively low. The CFRP sandwich panel was then subjected to combined gravity and lateral loading, which included seismic loads from a fictitious 5-story office building located in a region of high seismic risk. The results showed that CFRP sandwich panels are a viable option for use with floors, possessing sufficient strength and stiffness for meeting code prescribed design loads, while providing significant benefits over traditional construction materials.