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ICC B3: Scope and General Provisions (Scope, Definitions, Occupancy Classification)

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Scope and General Provisions (Scope, Definitions, Occupancy Classification)

Classification drives everything downstream

Scope provisions set the intent - safeguarding health, safety, and welfare - and the principle that the most restrictive requirement governs where the code conflicts with itself. Chapter 2 definitions then do heavy lifting: what counts as a story, a basement, a mezzanine, a fire area, an occupant load, and a change of occupancy. Chapter 3 assigns every space to a group: A (assembly), B (business), E (educational), F (factory), H (high hazard), I (institutional), M (mercantile), R (residential), S (storage), and U (utility), with sub-groups reflecting hazard and occupant vulnerability.

Traps

  • Assembly threshold. Assembly is generally about 50 or more occupants; below that, a similar room often falls into another group. The number, not the chairs, decides.
  • Accessory versus incidental. Accessory occupancies are subordinate and small in area and may be classified separately; incidental uses are listed in a table and must be separated from the primary occupancy by a rated assembly rather than being reclassified.
  • Mixed occupancy, two methods. Nonseparated occupancies apply the most restrictive height/area and fire-protection rules; separated occupancies require rated separation and allow each occupancy its own allowable area. Mixing the two methods mid-calculation is the classic error.
  • A "room" is not an occupancy. A storage closet in a business suite is not Group S unless it meets the code's criteria for separate classification.

Worked example / rule of thumb

A 9,000-square-foot office suite includes a 1,200-square-foot storage room accessed from the suite. Because it is subordinate in area and function to the business occupancy, it is normally an accessory use, and the whole floor is evaluated as Group B with the accessory area counted in the business area. Rule of thumb: classify by the principal use of the space, then ask whether the sub-use is accessory or incidental - that decides whether you rate a separation or simply add area.