This paper investigates the hierarchical cell structure (HCS) network consisting of macrocell and local area cell layers with partial cochannel sharing. We derive the ergodic capacity for each layer under cross-layer interference considering network-unplanned local area cell distribution. Based on the ergodic capacities, we also derive the area spectral efficiency (ASE) of the HCS network and investigate the optimum partial cochannel sharing that maximizes the ASE. It is confirmed that the maximum ASE is unchanged for a variable number of local area cells. Moreover, the spatial reuse benefit from the partial cochannel sharing is shown to be significant.