The inclusion of a washcoat model into monolithic catalyst simulations in order to create a one + one-dimensional framework using a minimum number of parameters is reviewed. All assumptions and numerical methods are presented while ensuring the prevention of excessive temperature excursions. Parametric studies illustrate the influence of model parameters (porosity, wall thickness, pore diameter) on the conversion of carbon monoxide during a light-off experiment using a uniform washcoat. Moreover, simulating a layered washcoat of dissimilar catalytic materials demonstrates a significant influence on conversion characteristics depending on the order of the layers. As a result, employing a washcoat requires careful consideration of the advection of the gas, the reactions occurring in each catalytic material, material layering, the diffusion properties of the different species, and the characteristics of the washcoat.