This paper proposes an efficient solution to the open problem of network planning for large-scale WLAN deployments. WLAN performance is governed by the CSMA-CA protocol, whose dynamic effects are difficult to capture. Accurate performance evaluation depends on simulations and takes time. A detailed analysis of dozens candidate designs with varying AP positions and channel assignments during network planning is therefore infeasible. In our solution, we first identify few good candidate designs using a multi-criteria optimization model, which features notions of cell overlap and station throughput. These candidate designs are taken from the corresponding Pareto frontier. In the second step, we evaluate the performance of the candidate designs by means of simulations. We apply our method to a realistic, large-scale planning scenario for an indoor office environment. The detailed simulations reveal important characteristics of the candidate designs that are not captured by the optimization model. The resulting performance differs significantly across the candidate designs. Hence, this approach successfully combines the benefits of mathematical optimization and simulations, yet avoiding their individual drawbacks.