Cobalt is a magnetic material that finds extensive use in various applications, ranging from magnetic storage to ultrafast spintronics. Usually, it exists in two phases with different crystal lattices, namely in hexagonal-close-packed (hcp) or face-centered-cubic (fcc) structure. The crystal structure of Co films significantly influences their magnetic and spintronic properties. We report on the thickness dependence of the structural and magnetic properties of sputter-deposited Co on a Pt seed layer. It grows in an hcp lattice at low thicknesses, while for thicker films it becomes a mixed hcp-fcc phase due to a stacking fault progression along the growth direction. The x-ray-based reciprocal space map technique has been employed to distinguish and confirm the presence of both phases. Moreover, the precise determination of Landé's g-factor by ferromagnetic resonance provides valuable insights into the structural properties. In our detailed experiments, we observe that a structural variation results in a nonmonotonic variation of the magnetic anisotropy along the thickness. This careful study reveals the fundamental physics, but also provides important insight for potential applications of thin Co films with perpendicular magnetic anisotropy.