Minecraft wiki entry for Before_Village_%26_Pillage, associated text: The dirt blocks the light from reaching the space below it, and so counts as a roof block. There is one "inside" space, covered by a roof block, on the right side of the image (represented by the red wool below) and zero on the left. Since one is more than zero, there are more covered spaces on one side of the door than there are on the other, and so the game counts this as a house: