We propose a model-based spatial layer bit allocation algorithm for H.264/scalable video coding (SVC) in this paper. The challenge of this problem lies in the fact that the rate-distortion (R-D) behavior of an enhancement layer is dependent on its preceding layers because of inter-layer prediction. To solve it, we first focus on the case of two spatial layers, derive the distortion and rate models of the dependent layer analytically, and develop a low-complexity bit allocation algorithm. It is shown by experimental results that the proposed two-layer bit allocation algorithm can achieve the coding performance close to the optimal R-D performance based on the full search method. Then, we extend this result to multilayer bit allocation by performing the two-layer allocation scheme recursively. Finally, we compare the performance of group of pictures-based and frame-based spatial layer bit allocation schemes at a fixed temporal resolution. The superior performance of the proposed spatial layer bit allocation algorithm is demonstrated using Joint Scalable Video Model reference software algorithm and two prior H.264/SVC rate control algorithms as the benchmarks.