The monitoring of urban areas (bridges, roads, dams, etc.) raises interest for permittivity estimation, in particular for failure detection. Radar sensors - ground or airborne-based - can offer solution for remote monitoring. This kind of environment can be characterized by double bounces scattering mechanism. Several methods have been developed for the case of a specular reflection using the Fresnel coefficients. In this paper, we recall these methods and propose a numerical solution to retrieve complex permittivities using double-bounces.