Major Building Use Classifications: 10 Groups and Mixed Use

The International Building Code sorts every structure into one of ten IBC occupancy classifications, each marked by a letter: A (Assembly), B (Business), E (Educational), F (Factory and Industrial), H (High Hazard), I (Institutional), M (Mercantile), R (Residential), S (Storage), and U (Utility and Miscellaneous). The group a building falls into is decided by how it is used and the hazards that use creates, and most groups split further into numbered subgroups with their own rules.

Why the Group Matters

A building’s occupancy classification controls its maximum allowable height, number of stories, and floor area. Those limits come from cross-referencing the group with the building’s construction type and whether it has an automatic sprinkler system.¹ The same classification also drives requirements for fire protection systems, exit routes, accessibility, and structural loads. A Group B office in fire-resistant construction can be much taller than a Group A assembly building in wood frame, because the risk profile is different.

Assembly (Group A)

Group A covers buildings where people gather for civic, social, or religious functions, recreation, eating and drinking, or waiting for transportation. Any assembly space with an occupant load of 50 or more falls here; anything smaller classifies as Group B.²

Assembly has five subgroups:³

A-1 covers performing arts and movie viewing with fixed seating, including theaters, concert halls, and television studios that admit audiences.
A-2 covers food and drink consumption: restaurants, bars, nightclubs, banquet halls, and casinos.
A-3 is the Assembly catch-all for worship, recreation, and amusement not covered elsewhere. Libraries, museums, courtrooms, bowling alleys, community halls, places of worship, and gymnasiums without spectator seating.
A-4 covers indoor sporting events with spectator seating, including arenas, skating rinks, and indoor pools or tennis courts with spectator areas.
A-5 covers outdoor activities: stadiums, grandstands, bleachers, and amusement park structures.

The line between A-3 and A-4 often comes down to spectator seating. A gymnasium where people exercise is A-3. Add bleachers for basketball and the same space becomes A-4, which carries stricter exit requirements because spectators are harder to evacuate than participants spread across a floor.

Business (Group B)

Group B covers office work, professional services, and service-type transactions. Banks, insurance agencies, government offices, and outpatient medical clinics all fall here.² Group B also absorbs small assembly spaces: any gathering room with fewer than 50 occupants classifies as B rather than A. College and university buildings generally sit in Group B as well.

Educational (Group E)

Group E applies to buildings used for education through the 12th grade when six or more people are present at any one time. Elementary, middle, and high schools land here, along with day care facilities serving more than five children older than 2½ years.² The heightened Group E protections are directed at younger populations who need more supervision during emergencies, which is why post-secondary buildings drop back to Group B.

Factory and Industrial (Group F)

Group F covers buildings where goods are manufactured, assembled, or repaired. Two subgroups split the group by hazard:²

F-1 (Moderate Hazard) covers operations involving materials that burn at a moderate rate, such as woodworking shops, textile mills, and furniture manufacturing.
F-2 (Low Hazard) covers operations involving noncombustible or low-combustibility materials, including brick or concrete block manufacturing, glass production, and metal fabrication.

If a factory’s materials exceed the code’s maximum allowable quantity thresholds for hazardous substances, the facility leaves Group F entirely and moves into Group H.

High Hazard (Group H)

Group H covers buildings that store, handle, or process materials posing a physical or health hazard in quantities above the maximum allowable amounts the code sets.⁴ Below those thresholds, a building with some hazardous materials can still classify as a less restrictive group, such as F or S, provided it meets the containment requirements for control areas.

The five High Hazard subgroups are organized by the type of danger:⁵

H-1 (Detonation Hazard) covers materials that can detonate, including certain explosives, Class 4 oxidizers, and unstable reactive materials. H-1 buildings face the strictest requirements in the entire code and cannot share a structure with any other occupancy group.
H-2 (Deflagration Hazard) covers materials that burn rapidly or create flash fire and explosion risks, including flammable gases, combustible dusts, and flammable cryogenic fluids.
H-3 (Physical Hazard) covers materials that readily support combustion but do not deflagrate or detonate, including consumer fireworks, flammable solids, and certain oxidizers in closed containers.
H-4 (Health Hazard) covers corrosive and highly toxic materials that threaten human health.
H-5 (Semiconductor Fabrication) covers facilities using hazardous production materials for semiconductor manufacturing, with specialized ventilation and containment requirements.

Institutional (Group I)

Group I covers buildings where occupants receive care, are under supervision, or have restricted freedom of movement. The common thread is that occupants may not be able to evacuate on their own, which drives significantly stricter fire protection and exit requirements.⁵

I-1 (Supervised Residential) covers more than 16 residents receiving custodial care in a supervised environment where they can respond to emergencies with limited or no assistance. Assisted living facilities, group homes, halfway houses, and residential rehabilitation centers.
I-2 (Medical Care) covers more than five people receiving 24-hour medical care who cannot evacuate on their own. Hospitals, nursing homes, and psychiatric facilities. This is among the most heavily regulated subgroups in the code.
I-3 (Restrained) covers more than five people held under security where occupants cannot freely leave. Jails, prisons, detention centers, and reformatories.
I-4 (Day Care) covers facilities providing care to more than five people of any age for fewer than 24 hours, including adult day care and child care where children are under 2½ years old.

Each subgroup further divides into numbered conditions based on how much help occupants need during evacuation.

Mercantile (Group M)

Group M covers buildings used for displaying and selling merchandise. Retail stores, department stores, drug stores, markets, and sales rooms classify here.² The line between M and S is activity: if customers walk through and browse products, it is mercantile; if goods sit waiting to ship, it is storage.

Residential (Group R)

Group R applies to buildings where people sleep, whether for one night or permanently. The subgroups split by length of stay, building size, and level of care:⁵

R-1 (Transient) covers short-term occupancy, including hotels, motels, and transient boarding houses with more than 10 occupants.
R-2 (Permanent, Multi-Unit) covers permanent residents in buildings with more than two dwelling units. Apartments, condominiums, dormitories, fraternities, sororities, and nontransient hotels.
R-3 (Permanent, Small-Scale) covers permanent residents in buildings with one or two dwelling units. Single-family homes, duplexes, small boarding houses, and care facilities with five or fewer residents.
R-4 (Supervised Residential, Small-Scale) covers 6 to 16 residents receiving custodial care in a supervised environment. Smaller-scale group homes, assisted living, and halfway houses. R-4 generally follows R-3 construction requirements with added care-environment provisions.

The line between R-4 and I-1 is the resident count. Once a supervised care facility passes 16 residents (staff excluded), it crosses out of Residential and into Institutional, which triggers substantially more demanding fire protection and construction requirements.

Storage (Group S)

Group S covers buildings used primarily for storing goods. Like Group F, it splits by hazard level:²

S-1 (Moderate Hazard) covers storage of items that burn readily, such as furniture, textiles, lumber, and tires.
S-2 (Low Hazard) covers storage of noncombustible or low-combustibility items, including metal parts, glass, and parking garages for motor vehicles.

Parking garages are the most common S-2 application. The classification determines whether the garage needs sprinklers and how many stories it can have for a given construction type.

Utility and Miscellaneous (Group U)

Group U is the catch-all for accessory structures that do not fit any other group. Private garages, carports, barns, agricultural buildings, and fences over six feet tall land here.² Because these buildings have low occupancy loads and limited hazards, the code imposes fewer requirements than it does for the other groups.

Buildings With More Than One Use

Most real buildings do not fit cleanly into a single group. A high-rise with retail on the ground floor, offices above, and a restaurant on top spans Groups M, B, and A-2. The IBC handles mixed-use buildings two ways.

In a nonseparated mixed-use building, no fire barriers divide the occupancies, and the entire building must comply with the most restrictive requirements of any group present. If one floor is Group A and the rest is Group B, the Group A rules govern the whole building.⁶

In a separated mixed-use building, fire-rated barriers divide each occupancy area and each area follows its own rules. This usually allows a larger overall building, because each group is evaluated on its own limits. High Hazard Groups H-2 through H-5 must always be separated from other occupancies regardless of the approach used elsewhere, and H-1 occupancies cannot exist in mixed-use buildings at all.⁶

Changing a Building’s Classification

Converting an old warehouse into apartments or turning a church into a restaurant means changing the occupancy classification, and the code does not treat that lightly. A change of occupancy classification requires approval from the local building official and a new certificate of occupancy.⁷

When a building moves to a higher-hazard group, it generally must be brought into compliance with the current code requirements for height, area, fire protection, structural loads, exits, accessibility, and mechanical and plumbing systems. Moving to an equal or lower-hazard group is less burdensome but still triggers review. Even a change within the same letter can force upgrades. Converting a bowling alley to a dance hall keeps the A-3 designation but may require different fire protection thresholds because the occupant load per square foot changes significantly.

Owners who skip the formal change-of-occupancy process risk having their certificate of occupancy revoked, facing code enforcement action, and losing insurance coverage if the building’s actual use does not match its approved classification.

1
International Code Council. 2021 International Building Code – Chapter 5 General Building Heights and Areas
2
International Code Council. International Building Code – Chapter 3 Occupancy Classification and Use
3
UpCodes. International Building Code 2024 – Chapter 3 Occupancy Classification and Use
4
UpCodes. High-Hazard Group H
5
International Code Council. International Building Code – Chapter 3 Occupancy Classification and Use
6
UpCodes. 508.3 Nonseparated Occupancies
7
International Code Council. 2018 International Existing Building Code – Chapter 10 Change of Occupancy