A method and apparatus for scalably encoding and decoding a video signal is provided. During encoding, prediction is performed on an image block to produce residual data of the image block, and the residual data of the image block is selectively coded into a residual difference from residual data of a block of a base layer, which spatially corresponds to the image block and is present in a base layer frame temporally coincident with a frame including the image block. Whether to code the image block into the residual difference is determined based on the difference between coding information (motion vectors or block pattern (CBP) information) of the image block and coding information of the corresponding block. Separate information indicating whether or not the image data has been coded into the residual difference is not transmitted to the decoder even if the image data has been coded into the residual difference.