This paper is aimed to describe the regions in the controller parameter space such that the given single-input single-output plant is stable and the H-infty norm of a related transfer function is less than a prescribed level. Two approaches are proposed: the first one is focused on covering the area of interest by intersection of admissible sets, while the second one enables to compute exactly its boundary via D-decomposition-like idea. The results are applicable for various low-order controllers.