  C$^3$ is an opportunity to realize an e$^+$e$^-$ collider for the study of
the Higgs boson at $\sqrt{s} = 250$ GeV, with a well defined upgrade path to
550 GeV while staying on the same short facility footprint. C$^3$ is based on a
fundamentally new approach to normal conducting linear accelerators that
achieves both high gradient and high efficiency at relatively low cost. Given
the advanced state of linear collider designs, the key system that requires
technical maturation for C$^3$ is the main linac. This white paper presents the
staged approach towards a facility to demonstrate C$^3$ technology with both
Direct (source and main linac) and Parallel (beam delivery, damping ring,
ancillary component) R&D. The white paper also includes discussion on the
approach for technology industrialization, related HEP R&D activities that are
enabled by C$^3$ R&D, infrastructure requirements and siting options.
