We extract the optical material parameters of a layer within a multilayer sample from terahertz (THz) time-domain spectroscopy data. To this aim, we derive a suited transfer function for multilayer structures in general and use the recently introduced spatially variant moving average filter to improve the accuracy of the algorithm. We demonstrate two applications of our method experimentally: the determination of the thickness and optical material parameters of an unknown layer on a substrate and the investigation of liquids in a known cuvette.