In this work we present a systematic study of the lateral (parallel to the wall) and normal (perpendicular to the wall) nanostructure of the electric double layer at a heterogeneous interface between two regions of different surface charges, often found in nanoscale electrochemical devices. Specifically, classical density functional theory (DFT) is used to probe a cation concentration range of 10 mM to 1 M, for valences of +1, + 2, and +3, and a diameter range of 0.15-0.9 nm over widely varying surface charges (between -0.15 and +0.15 C/m2). The DFT results predict significant lateral and normal nanostructure in the form of ion concentration oscillations. These results are directly compared with those from Poisson-Boltzmann theory, showing significant deviation between the two theories, not only in the concentration profiles, but also in the sign of the electrostatic potential.