RCW 58.28.060

Such surveyor must mark all corners of blocks or lots shown on the plat by substantial stakes or monuments, and must set stone or iron monuments at the points of intersection of the center lines of all the streets, where practicable, or as near as possible to such points, and their location must be shown by marking on the plat the distances to the block corners adjacent thereto. The top of such monument must be placed one foot below the surface of the ground, and in size must be at least six inches by six inches by six inches, and be placed in the ground to the depth of one foot.