The final configuration of the architectural shape is deeply conditioned by all constraints that the environmental context imposes. These, duly codified, can therefore generative elements that can be managed by the designer. Following a parametric approach, the architectural envelope is directly related to the constraints emerging from the urban context. In this paper we present the development and implementation of an application, which allows to generating three-dimensional models of buildings, directly within a three-dimensional geo-referenced environment. The application offers the user many innovative suggestions, associating the modelling ability, typical of the CAD systems, to the opportunity offered by the geo-referenced systems, to evaluate in real time the impact of the buildings directly on its environment, within which it can be generated and manipulated. The designer is able to explore several alternative solutions, according to formal requirements and style preferences. Three experiments are presented for different urban scenarios.