You only go to the IBC when the IEBC directs you there. Per IEBC Section 804.2.2, subparagraph 1, there is a reference to the IBC when the work area is required to be sprinklered per the IBC. IBC Section 903.2.1.2, subparagraph 2, requires a sprinkler system when the occupant load is greater than 100. Since you show an occupant load of 135 just in the dining room (assuming it is the dining room that is being altered), then you must provide a sprinkler system. One thing to keep in mind is the definition of "Work Area." It includes the area of all reconfigured spaces, so if two rooms have a combined area of 5,000 sq. ft. and the wall between them is moved, both spaces are reconfigured and the work area is 5,000 sq. ft. So, if in your dining room one portion is permanently reconfigured, then the entire dining room is essentially reconfigured and the work area includes the entire dining room floor area, which may be more than 50% of the total building floor area, thus triggering the sprinkler requirement of IEBC Section 804.2.2, subparagraph 2.
The reason I asked about what the occupant load was prior to the alteration is to look at this using the IEBC Prescriptive Compliance Method rather than the Work Area Compliance Method. If the occupant load hasn't changed or is reduced, then, per IEBC Section 403.1, then the building is "no less conforming to the provisions of the International Building Code than the existing building or structure was prior to the alteration." If the occupant load increases, then you would need to conform to the sprinkler requirements of the IBC. This is something I've discussed with ICC a few years ago for another restaurant project and they concured, since the occupant load for the alteration was less than the original occupant load.
The IFC has requirements for existing buildings (Chapter 11), but they only apply to existing buildings as they currently exist and not based on any alteration work. The sprinkler requirements of