When the bottom of the firebox opening is raised at least 8 inches above the top of the hearth extension, a hearth extension of not less than 3/8" thick brick, concrete, stone, tile or other approved noncombustible material is permitted.
Does the 3/8" thickness mean that the hearth extension should be that much above the floor surface or is that merely the thickness of the material?
Additionally, if this is on a slab, you can use 1/4" tile and still be compliant. The 3/8" minimum-thickness is assuming there’s a combustible material beneath the extension.