My code book tells me that a 2″ airspace must be maintained around a masonry chimney, which is not a problem so far as the framing is concerned. What about the sheathing, roof decking, siding, paper faced insulation, etc.?
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I believe that reads, "...excepting fro the use of trim materials when applied..."
There has been a gap between the reality and the code tho' An article in JLC about a year ago adressed this. There are a couple of new codes being written that may or may not be applicable in your area. Maybe it's one that includes the above.
The new codes also allow for less than one inch clearance where you have at least 12" of masonry and you are ____x distance from the firebox and throat.
My exact details may be not correct here as I am quoting from memory but the point is that if you do a little more research, you may find that these clearance issues are resolveable.
Speaking from a practicable view, you rarely see 2" maintained all the way. The point of danger comes where framing is in tight quarters with hot masonry and heat can build on it.
Generally, this does not happen all at once. Over time, the repeated overheating of wood fibres can carbonize it to the point that, instead of a 450 - 475 degree flash point, it can flash burn at a little over three hundred. If the fascia trim, for instance, begins to carbonize to that point, you will notice it first. The framing is hidden where you won't.