As stated in R502.1 of the ILECC, Additions to existing residential buildings, systems, or assemblies (walls, roofs, etc.) must follow the provisions of the ILECC as they would apply to new construction. However, this does not require unaltered portions of the existing building, systems, or assemblies to comply with the current code.

In short, this means anything that is new, or something existing being modified under the Additions scope of work must comply with current code requirements. Other existing systems and assemblies not included in the Additions scope of work and left untouched do not need to comply with the current code.