Deep donor and acceptor states in type I semiconductor heterostructures with a few coupled quantum wells are investigated using the approximation of a strongly localized impurity potential. The relations are established between the impurity energy spectrum and the impurity potential parameters. Quasistationary states that related to individual electron and hole subbands are considered. The possibility of exotic acceptor states arising through mixing of heavy and light holes in specially selected quantum well systems is shown. These states show a behaviour typical for one-dimensional systems. Such a behaviour is caused by the appearance of extreme loops in the dispersion relations of some of the hole subbands. Electron and hole scattering due to a localized impurity potential is investigated without any restriction on the doping profile. Also, deep donor and acceptor states in quantum well systems in an external electric field directed along the heterostructure axis are discussed briefly.