AEC definitions
Occupancy classification is the building code's category for how a space is used: assembly, business, educational, residential, and the other groups in the adopted code. The classification drives allowable height and area, fire separation, and egress. A change in how a space is used can be a change of occupancy, which is a permit problem, not an interior-design choice.
Definition
The International Building Code groups occupancies in Chapter 3. Local amendments adopt that text with edits. The drawings state the occupancy of each space. Mixed buildings either separate occupancies with rated walls or use a nonseparated strategy that applies the most restrictive rules. Accessory occupancies are small subsidiary uses. Getting the label wrong early makes the rest of the code analysis wrong. The plans examiner checks the label against the plan.
In depth
A room called a conference center may be assembly if the occupant load is high. A 'studio' may be residential. The code uses occupant load and the nature of the activity, not the leasing brochure.
Incidental uses, such as a furnace room, have their own separation rules and are not a new occupancy group.
Separated occupancies need the fire barrier the table requires. Nonseparated occupancies skip some of those walls and take the stricter height and area limits. The code sheet should say which strategy was used. Reviewers cannot infer it.
Accessory occupancies have size limits. Exceeding them means they are not accessory.
Egress width is calculated from occupant load, which comes from the function of the space. Changing a business area to assembly without revisiting the exits is how a renovation fails plan check.
Plumbing fixture counts also follow occupancy. The architectural code sheet should match the plumbing sheets.
Moving into an existing shell and changing the occupancy can trigger upgrades beyond the tenant's finishes.
The existing building code, as adopted, says which upgrades. Assume it applies until someone checks.
Examples
The code sheet classifies the office floors as Group B and the ground-floor restaurant as Group A-2, separated by the rated assembly the table requires.
A tenant wants a training room for 80 people in a business suite. The occupant load pushes the space toward assembly. Egress is recalculated before permit.
A storage mezzanine accessory to a factory is checked against the accessory-area limit.
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