City models by 3D CG (Computer Graphics) are important in promoting public participation for smart city, which will use solar photovoltaic (PV) generation. But, creating city models are labor intensive. In order to automate laborious steps, we proposed new technology by integration of a GIS (Geographic Information System) and CG. The proposed integrated system automatically generates 3D building models, based on building polygons or building footprints on digital maps, which show most building polygons' edges meet at right angles (orthogonal polygon). A complicated orthogonal polygon can be partitioned into a set of rectangles. The proposed integrated system partitions orthogonal building polygons into a set of rectangles and places rectangular roofs and box-shaped building bodies on these rectangles. In this paper, we propose to automatically generate 3D building models topped with double shed roofs attached by PV arrays. The sizes and positions, slopes of roof boards and under roof constructions are made clear by designing the top view and side view of a double shed roof house. For the application example of the developed system, we simulate the solar photovoltaic generation change of a city block by performing land readjustment and changing shape of buildings, that is, ordinary roof house or double shed roof house suitable for greater PV generation. Our simulation reveals that double shed roof houses have greatly improved the solar photovoltaic generation.