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. You are responsible for demonstrating that the wall is in fact an extension per this section, so we recommend discussing this with the CEC Hotline (800) 772-3300.
Assuming you do meet the exception, we have a checkbox that allows you to indicate that exception at the Wall level of the building tree. Just be aware, this is a specific exception that only applies to those walls that meet the CEC definition of an extension.
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