Rather than merely dealing with robot localization and mapping in 3D environments, this paper tackles a special topic which could be denominated as-is 3D building modeling with robots. We present a method with which to carry out the automatic 3D scanning of furnished buildings with the aim of obtaining complete and in-depth information regarding the principal visible structural components of an indoor environment (walls, floors and ceilings). This is an essential stage in the automatic creation of as-built BIM models that greatly facilitates the detection of the structure of a building. Our system, which is essentially composed of a mobile robot and a 3D laser scanner, autonomously navigates indoor environments that are occluded and cluttered. The approach can be considered as a structural-component based 3D mapping in which the robot moves to certain positions that are calculated using a new Next Best Scan (NBS) algorithm. We justify the contributions of our work in this research field by means of a broad theoretical comparison with other related methods. Key issues such as the complexity of the scene, occlusion, hypotheses, limitations and autonomy are discussed in the paper. This system has been tested in real and simulated environments. Complex scenes composed of several adjacent non-regular rooms (i.e., concave/convex rooms) have been tested, with promising results.