To support spatial scalability, the scalable extension of H.264/AVC (SVC) uses video cropping or uniform scaling to downscale the original higher-resolution (HR) sequence to a lower resolution (LR) sequence. Both operations, however, will cause critical visual information loss in the resized frames. To address the problem, we propose a low-overhead content-adaptive spatial scalability SVC (CASS-SVC) coder consisting of three main modules: a mosaic-guided video retargeter, a side-information coder, and a non-homogeneous inter-layer predictive coder. The proposed video retargeting scheme first constructs a panoramic mosaic for each video shot to obtain a compact shot-level global scaling map which is then used to derive the scaling maps of individual frames in the shot. The side information required for the non-homogeneous scaling, including the global scaling maps and the spatial corresponding positions of individual frames to the panoramic mosaic, are then efficiently coded by the side-information coder. The non-homogeneous interlayer prediction coding tools are used to provide good predictions to reduce the bitrates for coding the HR frames. Our simulation results demonstrate that, compared to existing CASS-SVC coders, our method cannot only well preserve subjective quality of important content in the LR sequence, but also significantly improves the coding efficiency of HR sequence.