Certain building types carry extra requirements beyond the base occupancy rules, collected in IBC Chapter 4. These include high-rise buildings (403), with added fire protection, structural, and egress provisions once a building exceeds 75 feet to the highest occupied floor; atriums (404), requiring smoke control and specific separation; covered and open mall buildings (402); and detailed rules for institutional, motor-vehicle-related, and hazardous occupancies.
The trigger is often a single threshold — building height, a multi-story open space, or a use type — that pulls in a whole set of additional requirements.
The common mistake is missing that a design has crossed into a special-occupancy category — for example, a multi-story lobby that meets the definition of an atrium and now needs smoke control, or a building that just tops the high-rise threshold. Reviewers check the height and the space types against Chapter 4 triggers, because these requirements are substantial and much cheaper to design in early than to retrofit.
This guide describes the model code for general understanding and is not a substitute for the adopted code and amendments enforced by your local authority having jurisdiction. Verify all figures against your jurisdiction's codes of record.