This paper presents an advance in a method to evaluate the diffuse solar potential on facades in the urban environment. The proposed method is suitable to work in both the isotropic and anisotropic sky conditions. It employs the 2.5D geo-information model, combined with the forward ray-tracing technique that has been found efficient in urban analysis due to its hyperpoints-independent characteristic. For the anisotropic sky conditions, a well-known solar radiance model is integrated to include the atmosphere-dependent scenarios by considering the solar diffuse fraction, clearness index and the position of sun in sky. To illustrate this work, an urban case study performed under various sky conditions is presented. The results obtained are compared and discussed.