In the context of ground based Radar or wireless communications, non line of sight situations occuring in suburban type built environments are particularly demanding regarding the accuracy of field simulations. This communication presents new developments of the frame-based Gaussian beam shooting method to address some of the diffraction problems encountered in the presence of buildings. The presentation is focused on the problem of back-scattering by the bottom and the top of a building, which is addressed through incident beam selection, image frame and frame redecomposition. Numerical results obtained with this algorithm are compared to results obtained with the Parabolic Equation and with reference results