Occupancy classification is the starting point of any code analysis. Before you can determine allowable building height, area, construction type, or egress requirements, you need to know what occupancy group or groups apply to the building. The IBC organizes all buildings into occupancy groups based on their purpose, function, and the relative risk they pose to occupants.

This isn't just an academic exercise. The occupancy classification drives nearly every downstream code decision: fire-resistance ratings, sprinkler requirements, means of egress capacity, interior finish restrictions, and plumbing fixture counts all trace back to what group letter sits on your code analysis sheet.

For the ARE, you need to distinguish between the ten major occupancy groups (A, B, E, F, H, I, M, R, S, U) and their subgroups. You also need to understand what happens when a single building contains more than one occupancy. The IBC provides three approaches for handling mixed occupancies: accessory use, nonseparated occupancies, and separated occupancies. Each approach carries different requirements for fire barriers, area calculations, and height limitations.

Getting the classification right at the programming phase shapes everything that follows. A misclassified space can blow up your area calculations, trigger unexpected sprinkler requirements, or force construction type upgrades that wreck the budget.