The possibility of stacking two-dimensional (2D) materials into van der Waals (vdW) heterostructures has recently created new opportunities for band structure engineering at the atomic level. However, despite the weak vdW interaction, controlling the electrostatic potential governing the band lineup at the 2D interfaces is still posing a significant challenge. Here, we demonstrate that 2D Janus monolayers, possessing an intrinsic out-of-plane dipole moment, can be used to control the band alignment at semiconductor-semiconductor and metal-semiconductor interfaces in a highly predictive manner. Using density functional theory (DFT), we calculate the band structure of a wide range of different vdW interfaces. We find that upon insertion of a Janus structure the band line-ups and Schottky barriers can be controlled to high accuracy. The main result of this work is that the out-of-plane dipole moment of the Janus structure changes little upon insertion in the interface. As a consequence, the effect on the electrostatic potential at the interface can be predicted from the properties of the freestanding Janus structure. In addition to this, we predict 47 stable Janus monolayers, covering a wide range of dipole moments and band edge positions and thus providing a comprehensive library of 2D building blocks for manipulating the band alignment at interfaces.