Hearth extension

agreed, but if we are talking about a hearth extension that whose main purpose is for embers shooting out, any substrate does not get very hot at all in front of the stove in a extended hearth scenario, the finish material, yes, it gets warm, pretty warm. when my stove is 600d and i feel the floor in front, it's only warm.
any other application, i agree that durock is the safe choice.

I agree in principle, but the inspector is gonna want to see full compliance to the minimum R-value out to the minimum required dimension from the front of the loading door. If that R-value is met up to that dimension, then it's fine. And yeah - anything out further is purely for added protection. Even my limited personal experience tells me that you should not skimp on this dimension - I had a huge cinder come shooting out at me last night - burned a big spot on my shiny new bellows Better that than my hardwood floors...

the building inspector ?..yeah, gotta keep him happy. whatever he wants