In this work, local mesh adaptivity for the time harmonic Maxwell equations is studied. The main purpose is to apply a known a posteriori residual‐based error estimator from the literature and to investigate its performance for a Y‐beam splitter setting. This configuration is an important prototype for the design of optical systems within the excellence cluster PhoenixD. Specifically, the branching region is of interest and requires a high accuracy of the numerical simulation. One numerical example shows the performance of our approach.