Minecraft 1.9.2 Flat Word Block Height Limit

I was building a kingdom in a flat world on Minecraft 1.9.2 and when I tried placing the twentieth block, (height-wise) their was an error message stating, "Height limit for building is 256 blocks." I don't understand this because it is only my twentieth block high. The faster someone could respond the better so then I can continue to work on my building.