We propose elastic neutron powder diffraction experiments on newly discovered perovkite bismuthates, (Ca,K)BiO3. Known as one of the earliest high-Tc oxide superconductors, bismuthates have provided a model system for studying the interplay between a charge density wave caused by Bi-O bond length disproportionation, and superconductivity. Recently, octahedral distortions driven by small A-site ions have been suggested to be a control knob of electronic as well as superconducting properties; however, its detailed origin is yet to be understood. High-resolution diffraction experiments at E9 will enable us to precisely determine the crystal structure of CaBiO3 and its superconducting doped compounds, both of which are expected to be highly distorted due to the small ionic radius of Ca. Our results will be important to understand the origin of superconductivity and its link to structural distortions in bismuthates, which can in turn lead to a design principle for new superconductors.