Yes, that is an addition, however, gross and net is really used within Chapter 10, FBC-Bldg to assist in occupancy loading. Saying that, if something is not defined within one volume of a code, then go to the prime (Bldg) to see if it is defined.
Look first at the definition of Area, Bldg - the last sentence reads "areas of the building not provided with surrounding walls shall be included in the building area if such areas are included within the horizontal projection of the roof or floor above." The definition of Floor Area, gross has similar verbiage.
In the FBC-Existing, Section 603.1 scope of a Level II Alteration includes the reconfiguration of space,......, the reconfiguration or extension of any system, ...... In enclosing a carport or a garage, that would be extending a wall system within the existing floor space due to the roof system being there.
This an alteration creating additional habitable space, therefore, an addition. Be aware of the other code requirements bringing this area into a habitable use.
