Building demolitions are often triggered by reasons independent of their structural conditions, causing the disposal of large quantities of reinforced concrete in good condition. Today, this concrete is, at best, crushed into aggregates, causing the loss of its embodied structural, technological, and historical qualities. Yet, techniques exist to cut large concrete pieces with circular saws, and the latter can be reused as-is in new structural systems. Those new systems typically require significantly fewer new materials and reduce both waste generation and embodied carbon of the new project, improving the sustainability and circularity of the new construction. As floors typically account for a large share of a structure’s embodied carbon, this paper explores new design options for floors reusing large pieces cut from cast-in-place concrete structures. The study investigates four ways to cut floor slabs and compares nine conceptual floor systems made of reused cut RC elements. After comparing the systems via a set of transdisciplinary design principles merging architectural engineering, project management and sustainability, a system stood out from the comparison: a floor system that reuses flat slab elements lying on transverse girders. The construction of a 30-m2 mock-up of this system confirmed the technical feasibility, construction and disassembly speed, and environmental benefits.