Section 150.2 of the code specifically allows small additions that comply Prescriptively to be built with either 2×4 R-15 or 2×6 R-19 walls in cases where that wall is an extension of the existing wall, or the siding is not being removed. You are responsible for demonstrating that the wall is in fact an extension per this section, or has the siding remaining so we recommend discussing this with the CEC Hotline (800) 772-3300.
Assuming you do meet the exception, we have a pulldown that allows you to indicate that exception at the Wall level of the building tree. This exception will then be documented on the CF1R.
← How do I indicate the Prescriptive Addition Exception that allows for Walls to avoid Rigid Insulation?