What is your opinion of box extenders like the one pictured on this link? When a wall or ceiling is sheeted over, do these devices satisfy the requirements of article 314.20? I've always thought no. If the box is mounted in noncombustible material, and is less than 1/4" back-set, they wouldn't be necessary at all, and if the box is mounted in combustible material it must be mounted flush. If the box is set back more than 1/4" in noncombustible material, or less than flush with combustibles, I've always assumed that the box needed to be moved out. I realize that in most cases the AHJ would probably allow this, but is it technically a valid fix? Article 314.20 has no exceptions.
Matt box extender