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.

This would be the case when the fireplace hearth is at the same level as the new floor, although not when the hearth is substantially higher than the floor level.

You can build a stud wall around the chimney. If the studs are wood, there must be a 2-inch space between the studs and the bricks. Because steel studs are noncombustible, they can be installed against or attached to the bricks, as long as there are no combustibles, including drywall, within 2 inches of the chimney. Instead of framing out around the masonry, one simple solution is to parge the chimney with a coat of stucco, or to screw cement backerboard directly to the chimney. The cement board can be finished with a skim coat of drywall compound. If stucco or cement board is installed on the chimney, the surface can be painted, but should never be wallpapered.

Finally, remember that your local code may have more stringent requirements than the 2-inch clearance required by BOCA. For example, diagram 3610.4.7a of the Massachusetts Building Code requires a 4-inch clearance, not a 2-inch clearance, between combustibles and the outside of the brick.