A. Stephen Bushway, mason and venting system specialist, responds: Unless the thickness of the chimney wall is at least 8 inches, most codes prohibit combustibles in direct contact with a chimney. The only exceptions are for pieces of wall trim and roof sheathing.
In the case of your addition, you will be creating an interior fireplace. In the area directly behind and around the firebox, all combustibles must be kept at least 2 inches away from the outside of the brick. Because of this requirement, in most cases the floor framing of the addition must also be kept at least 2 inches away from the chimney.
to read the full article.