The capabilities of Interferometric SAR (IFSAR) to provide Digital Elevation Model is well known and have been highlighted with the recent Shuttle Radar Topographic Mission. Nevertheless, layovers, shadows, multiple bounces and surfaces discontinuity strongly reduce the accuracy of the extracted shapes in urban areas with high-resolution systems. Due to the number and the complexity of uncontrolled interactions at work, we choose to use simulations to study the layover produced by buildings. An interferogram simulator developped at CNES, IRIT and CS, which is called 2SIR, can achieve those. This tool has been designed in order to simulate realistic interferograms (and its pair of radar images) in such complex situations as urban areas. C. Prati et al. have shown that a slope induces a spectral shift of the ground response. Although it generates a loss of coherence, it can be used to partially separate layover and non layover areas as they done with ERS images. In high- resolution, the spectral shift is insufficient to apply such a filtering. However, this technique can be achieved in urban area with certain restrictions to extract the shape of buildings.