The ground-penetrating radar (GPR) method has been used for many years in archaeological research. However, this method is still not widely used in studies of past architecture. The biggest problem with the implementation of the GPR method at such sites is usually connected with extensive debris layers, plant cover and standing relics of walls and other features that restrict the available measurement area. Despite of these, properly performed GPR surveys, even on a small area, can yield significant information concerning underground architectural relicts. Moreover, the results of GPR profiling integrated with historical and archeological data allow for three-dimensional reconstruction of the examined architectural monuments and in the next step, they track architectural transformations. Relics of the Romanesque St. Peter monastery, located in the northern part of the Island of Rab, is a good example of the successful GPR survey. Results of the performed geophysical reconnaissance in conjunction with the query of archival materials made it possible to visualize a spatial (3D) appearance of three main phases of the site architectural development, despite a very limited area available for geophysical survey. According to the authors, such a comprehensive approach should be a standard in contemporary geophysical research focused on relics of the past architecture.