Section 2.1: Occupancy Classification and Use

Occupancy Classification: The Foundation of Plan Review

Determining a building's occupancy classification is the essential first step in a plans review, as it establishes the baseline requirements for fire suppression, height, area, and egress. Under Chapter 3 of the 2024 International Building Code (IBC) and Chapter 2 of the 2024 International Fire Code (IFC), occupancy classification categorizes a structure based on its intended use, the characteristics of the occupants, and the internal hazards. A plans examiner must verify this classification immediately, as it dictates height and area limits (IBC Chapter 5), construction types (IBC Chapter 6), fire protection systems (IBC/IFC Chapter 9), and egress layouts (IBC/IFC Chapter 10).

All structures are classified into one or more of the occupancy groups detailed below in accordance with IBC Section 302.1.

Group A: Assembly Occupancies

Assembly occupancies are designed for civic, social, religious, dining, or transit gatherings. These spaces pose unique safety risks due to high densities, occupant unfamiliarity, and panic hazards. Group A is divided into five subgroups:

• Group A-1: Spaces with fixed seating for viewing performing arts or films (e.g., movie theaters, concert halls). Plans examiners must evaluate aisle spacing and exit access travel distances under low-light conditions.
• Group A-2: Spaces for food and drink consumption (e.g., nightclubs, bars, restaurants). High crowd density and cooking hazards trigger automatic sprinkler requirements at lower thresholds compared to other assembly groups. Under IBC Section 903.2.1.2, a sprinkler system is required if the occupant load is 100 or more, or the fire area exceeds 5,000 square feet.
• Group A-3: Spaces for worship, recreation, or amusement not classified elsewhere (e.g., places of worship, art galleries, gyms without spectator seating).
• Group A-4: Spaces for viewing indoor sporting events with spectator seating (e.g., arenas, indoor pools with bleachers).
• Group A-5: Outdoor activities and spectator viewing (e.g., stadiums, grandstands).

Small Assembly Exceptions

To prevent small conference rooms or accessory spaces from triggering costly assembly requirements, the code provides two critical exceptions under IBC Section 303.1.1 and 303.1.2:

1. Occupant Load < 50: A room used for assembly with an occupant load of fewer than 50 persons is classified as a Group B occupancy or as part of the main occupancy.
2. Accessory Spaces: An assembly space accessory to another occupancy that is less than 750 square feet is classified as Group B or as part of the main occupancy.

Group B: Business

Business occupancies are used for office, professional, or service-type transactions. Examples include offices, banks, post offices, testing laboratories, and outpatient clinics. Higher education classrooms (colleges and universities) are classified as Group B, not Group E. Ambulatory care facilities where patients receive outpatient medical care but are not rendered incapable of self-preservation on a 24-hour basis are also classified as Group B, subject to the additional safety rules in IBC Section 422.

Group E: Educational

Group E occupancies include buildings used by six or more persons at one time for educational purposes through the 12th grade (K-12). This group also covers daycare facilities. Daycare facilities caring for more than five children older than 2.5 years are classified as Group E. If the facility cares for five or fewer children of any age in a single-family dwelling, it is classified under the International Residential Code (IRC) or as Group R-3. If the facility cares for more than five children who are 2.5 years or younger, it must be classified as Group I-4.

Group F: Factory & Industrial

Factory occupancies include buildings used for manufacturing, packaging, or processing materials not classified as hazardous:

• Group F-1 (Moderate-Hazard): Factory uses involving combustible materials (e.g., furniture manufacturing, paper mills).
• Group F-2 (Low-Hazard): Factory uses involving noncombustible materials (e.g., metal fabrication, glass manufacturing).

Group H: High-Hazard

Group H occupancies involve the manufacturing, processing, or storage of materials constituting physical or health hazards in quantities exceeding the Maximum Allowable Quantities (MAQs) in IBC Table 307.1(1) and Table 307.1(2). Group H is subdivided as follows:

• Group H-1: Detonation hazards (e.g., explosives).
• Group H-2: Deflagration or accelerated burning hazards (e.g., flammable gases under pressure).
• Group H-3: Physical hazards supporting combustion (e.g., flammable solids, oxidizers).
• Group H-4: Health hazards (e.g., highly toxic or corrosive materials).
• Group H-5: Semiconductor fabrication facilities using hazardous production materials (HPM) in cleanrooms.

Group I: Institutional

Institutional occupancies include buildings where people receive care/supervision in a residential environment or are detained. Occupants typically have physical or mental limitations affecting self-preservation or are physically restrained.

• Group I-1: Facilities where more than 16 occupants reside on a 24-hour basis and receive custodial care, but are capable of self-preservation with assistance (e.g., assisted living, group homes).
• Group I-2: Facilities providing 24-hour medical or nursing care for more than five occupants who are incapable of self-preservation (e.g., hospitals, nursing homes).
• Group I-3: Facilities inhabited by more than five persons under restraint or security (e.g., prisons, jails).
• Group I-4: Daycare facilities providing care for more than five clients who are incapable of self-preservation on a less than 24-hour basis (e.g., infant daycares).

Group M: Mercantile

Mercantile occupancies include buildings used for the display and sale of merchandise. Examples include retail stores, department stores, and drugstores. The fire hazard is driven by the volume of combustible merchandise, high-shelving configurations, and packaging materials.

Group R: Residential

Group R occupancies include buildings used for sleeping purposes when not classified as Institutional.

• Group R-1: Transient sleeping accommodations (occupants staying < 30 days, e.g., hotels, motels).
• Group R-2: Permanent residential occupancies containing more than two dwelling units (e.g., apartments, dormitories).
• Group R-3: Permanent residential occupancies not fitting into R-1, R-2, or R-4, such as single-family homes or small care facilities housing 5 or fewer clients.
• Group R-4: Residential care or assisted living facilities housing between 6 and 16 occupants.

Group S & Group U

• Group S (Storage): Storage of materials not classified as hazardous. Divided into Group S-1 (Moderate-Hazard: combustible items like lumber or furniture) and Group S-2 (Low-Hazard: noncombustible items or parking garages).
• Group U (Utility): Accessory structures not classified elsewhere (e.g., carports, greenhouses, sheds, agricultural buildings).

Multiple Occupancies and Mixed-Use Buildings

When a building contains two or more occupancy classifications, the plans examiner must evaluate the structure under one of three options under IBC Section 508:

1. Accessory Occupancies (IBC Section 508.2): Ancillary to the main occupancy and limited to 10% of the building area of that story. Egress must comply with the accessory use, but height and area limits are based on the main occupancy, and generally no fire-resistance separation is required.
2. Nonseparated Occupancies (IBC Section 508.3): No fire-resistance-rated separation is required. The height and area limitations for the entire building are based on the most restrictive occupancy present. Egress and fire protection rules apply to each space based on its occupancy, but the most restrictive sprinkler/alarm thresholds apply to the entire fire area.
3. Separated Occupancies (IBC Section 508.4): Occupancies must be separated by fire barriers or horizontal assemblies in accordance with Table 508.4. Each separated portion is governed by its own height, area, and fire protection requirements. For example, a Group R-2 occupancy separated from a Group B occupancy requires a 2-hour fire-resistance rating if the building is unsprinklered, or a 1-hour rating if the building is fully sprinklered.

Worked Scenario: Mixed-Use Plan Review

Consider a plans review for a new three-story building. The first floor is proposed as a retail store (Group M, 5,000 square feet), and the second and third floors are proposed as residential apartments (Group R-2, 5,000 square feet per floor). If the design professional selects the nonseparated occupancy option (IBC Section 508.3), no fire-resistance-rated wall separation is required between the retail store and the apartments. However, the entire building must meet the most restrictive height and area limitations of either Group M or Group R-2. Furthermore, since Group R-2 requires an automatic sprinkler system throughout (IBC Section 903.2.8), the entire building—including the first-floor mercantile space—must be sprinklered. If the designer selects the separated occupancy option (IBC Section 508.4), a fire barrier must be constructed to separate the first-floor Group M from the second-floor Group R-2. Consulting IBC Table 508.4, the required rating is 1 hour in a sprinklered building. Egress from the residential units must be completely separated from the mercantile area, but the allowable area calculations can be performed independently for each occupancy.

Mixed-Use Classification Options Summary Table