A method of decoding coding blocks of a coding tree unit in an image. The method comprises obtaining the coding tree unit in the image from a bitstream, the coding tree unit having chroma channels and a luma channel; and determining luma split options for the luma channel of the coding tree unit according to dimensions of a region of the coding tree unit. The method also comprises determining chroma split options for the chroma channels of the coding tree unit according to dimensions of a region of the coding tree unit, the chroma split options being different to the luma split options, the allowable chroma split options resulting in chroma intra predicted blocks with a minimum size of 16 samples; and decoding the coding blocks by determining flags from the bitstream to select one of the determined luma split options and one of the determined chroma split options.