With the rapid increase of complexity and volume of 3D building models, industries such as digital games and computer-aided design face considerable challenges. One feasible solution is to generate low-polygon models for complex building meshes in a relatively short time to apply in a level of detail (LOD) algorithm. We propose an efficient building model simplification algorithm with opening detection, aiming to realize an extremely low polygon count while preserving their visual features. Our algorithm has three steps. We detect and construct the opening structures presented in the building model to incorporate in the subsequent carving process; improve the approach to generate a visual hull that represents the fundamental boundary of the building model and carve out details with high efficiency; and apply planarization and simplification algorithms to obtain the simplified model. Our algorithm can generate highly simplified building models while preserving important opening details to serve as the roughest level in the LOD hierarchy. Experimental results demonstrate that our algorithm efficiently handles building models with complex opening structures and outperforms other recent simplification algorithms.