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) one. Both operations, however, will cause critical visual information loss in the retargeted frames. The content-adaptive spatial scalability SVC coders (CASS-SVC) use non-homogeneous scaling to avoid critical information loss, which, however, requires to send additional side information to signal the decoder, thereby degrading coding efficiency significantly. To address the problem, we propose a low-overhead 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. Our experimental 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.