Carefully extracting reinforced concrete (RC) elements from soon-to-be demolished structures and reusing them directly as load-bearing elements in new buildings is an emerging circular low-carbon resource-management strategy. As floor construction typically accounts for a large share of a building’s upfront carbon footprint, designing floors with reused RC elements is a promising, yet little explored, approach to lower a building’s embodied carbon. This paper presents the concept, design, construction and assessment of a new load-bearing floor system for an office building made with reused saw-cut RC pieces and reused steel profiles. The system reuses the existing properties of widely discarded construction materials – RC and steel – and is dismountable. To demonstrate the system’s technical feasibility and assess its structural and environmental performance, a 30-m2 prototype – FLO:RE – is designed, built with elements reclaimed from local demolition sites, tested and finally dismantled. Reclaimed material property testing and prototype load testing confirm the structural-design safety. A Life-Cycle Assessment shows unprecedentedly low upfront embodied carbon, with results as low as 15 to 5 kgCO2e/m2, i.e., 80–94% reductions compared to conventional new RC flat slabs. This research demonstrates the untapped technical and environmental potential of reusing saw-cut RC elements in bending in structurally performant floor systems. Through this novel ultra-low-carbon solution, the study supports the efficient use of existing resources and calls for considering soon-to-be demolished RC and steel structures as potential mines of suitable quality materials ready to be reused locally.