The IEBC has 3 different paths a Designer gets to choose use not the building department. A designer cannot mix the different options.
301.3.1 Prescriptive compliance method.
Alterations, additions and changes of occupancy complying with Chapter 5 of this code in buildings complying with the International Fire Code shall be considered in compliance with the provisions of this code.
301.3.2 Work area compliance method.
Alterations, additions and changes of occupancy complying with the applicable requirements of Chapters 6 through 12 of this code shall be considered in compliance with the provisions of this code.
301.3.3 Performance compliance method.
Alterations, additions and changes of occupancy complying with Chapter 13 of this code shall be considered in compliance with the provisions of this code.