Owing to their simplicity, brick layer models are frequently used to analyze the impedance spectra of ceramics. As a matter of fact, such models do not represent the real microstructure of the materials. In this contribution, we investigate how deviations from that idealized model influence the overall impedance for low-conducting grain boundaries. Finite element calculations reveal that even homogeneous grain boundaries can cause distinct differences between the true grain boundary impedance and the brick layer values. In various relevant cases, however the deviations are not very pronounced and may even partly compensate such that the brick layer model can still be a reasonable approximation in many cases.