Polarimetric interferometric synthetic aperture radar (SAR) is more advantageous than either polarimetric or interferometric SAR alone in extracting information from vertical object structure. Generally, for spaceborne SAR, its resolution is low and several targets may be contained in the same resolution cell. Based on the characteristics of spaceborne SAR, a built-up area scattering model (BASM), considering buildings and vegetation in the same resolution cell, is proposed to extract building parameters using L-band polarimetric interferometric SAR data. For single-baseline polarimetric interferometric SAR, the scattering component related to the natural targets could be removed from polarimetric coherency matrices and polarimetric interferometric matrix. Then building parameters could be estimated using the BASM after detecting built-up areas. The BASM is validated using ALOS PALSAR L-band polarimetric interferometric SAR data and experimental results indicate that building parameters could be effectively extracted by the proposed BASM.