Code note about exterior walls

As you probably recall, I’m trying to build this house as close to 1000 square feet as possible, because that is the largest allowed by county codes for 2nd dwellings on a single parcel. However is 1000 square feet defined? Well, it’s everything inside the exterior walls. So, if you have thick walls, you lose space to wall thickness. That’s a bummer for those who want to insulate well, since that means thick walls and thus less square footage. However, my architect brother-in-law said that he believed the code said the exterior walls end where the structure ends, so anything outside the exterior wall structure doesn’t count towards wall size. Specifically, if you have insulation, sheathing, siding, etc outside the structure, you ought to be good to go. Well, I did some code searching today and here’s what I found in Chapter 2 of the International Building Code:

EXTERIOR WALL. A wall, bearing or nonbearing, that is used as an enclosing wall for a building, other than a fire wall, and that has a slope of 60 degrees (1.05 rad) or greater with the horizontal plane. 

EXTERIOR WALL COVERING. A material or assembly of materials applied on the exterior side of exterior walls for the purpose of providing a weather-resisting barrier, insulation or for aesthetics, including but not limited to, veneers, siding, exterior insulation and finish systems, architectural trim and embellishments such as cornices, soffits, facias, gutters and leaders. 

EXTERIOR WALL ENVELOPE. A system or assembly of exterior wall components, including exterior wall finish materials, that provides protection of the building structural members, including framing and sheathing materials, and conditioned interior space, from the detrimental effects of the exterior environment.

So, “Exterior wall” isn’t entirely clear whether that includes insulation, but if you hop down to “Exterior Wall Covering”, you see that that includes anything on the exterior side of the exterior wall, including insulation, siding, trim, etc. Then they have a separate term, “exterior wall envelope”, to include exterior wall plus all the coverings. (And just as a reference for myself, Island County uses the 2012 International Building Code, which can be found referenced here:, where it states “The 2012 International Building Code is in effect”.)

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