CuAgBiI5 is synthesised as powder, crystals, and thin films. The structure consists of a 3D octahedral Ag+/Bi3+network as in spinel but the tetrahedral interstitials occupied by Cu+ differ from those of a spinel. The 3D octahedral network of CuAgBiI5 allows us to identify a relationship between octahedral site occupancy (composition) and octahedral motif (structure) across the whole CuI-AgI-BiI3 phase field, giving the ability to chemically control structural dimensionality. To investigate composition-structure-property relationships, we explore how the basic optoelectronic properties of CuAgBiI5 compare to those of Cu2AgBiI6 (which has a 2D octahedral network) and reveal a surprisingly low sensitivity towards dimensionality of the octahedral network. The absorption onset of CuAgBiI5 (2.02 eV) barely changes compared with that of Cu2AgBiI6 (2.06 eV) indicating no obvious signs of an increase in charge confinement.  Such behaviour contrasts with that for lead halide perovskites which show clear confinement effects upon lowering dimensionality of the octahedral network from 3D to 2D. Changes in PL spectra and lifetimes between the two compounds mostly derive from difference in extrinsic defect densities rather than intrinsic effects. While both materials show good stability, bulk CuAgBiI5 powder samples are found to be more sensitive degradation under solar irradiation compared to Cu2AgBiI6.