A detailed diffraction study of Ca₁₀Cr₇O₂₈ is presented which adds significant new insights into the structural and magnetic properties of this compound. A new crystal structure type was used where the a and b axes are doubled compared to previous models providing a more plausible structure where all crystallographic sites are fully occupied. The presence of two different valences of chromium was verified and the locations of the magnetic Cr⁵⁺ and non-magnetic Cr⁶⁺ ions were identified. The Cr⁵⁺ ions have spin-½ and form distorted kagome bilayers which are stacked in an ABC arrangement along the c axis. These results lay the foundation for understanding of the quantum spin liquid behavior in Ca₁₀Cr₇O₂₈ which has recently been reported in Balz et al (2016 Nat. Phys. 12 942).