Occupancy Classification Chart: Groups, Subgroups, and Mixed Use

The International Building Code sorts every structure into one of ten occupancy classifications, labeled A, B, E, F, H, I, M, R, S, and U, based on what happens inside the building and the hazard that use creates for the people in it.¹ Most of these groups split further into numbered subgroups. The classification a building carries controls its fire protection, exit design, allowable height and area, and construction type, so matching the label to the actual use is not paperwork. It is the difference between a legal building and one that can be shut down.

The Ten Occupancy Groups

IBC Section 302.1 lists every group as follows:²

Group A (Assembly). Subgroups A-1 through A-5. Spaces where people gather for entertainment, dining, worship, or similar purposes.
Group B (Business). Offices, banks, professional services, and government buildings.
Group E (Educational). Schools serving students through twelfth grade, plus day care for more than five children over two-and-a-half years old.
Group F (Factory/Industrial). Subgroups F-1 and F-2. Manufacturing and fabrication, split by fire hazard.
Group H (High Hazard). Subgroups H-1 through H-5. Buildings that store or use materials posing explosion, fire, or health risks.
Group I (Institutional). Subgroups I-1 through I-4. Facilities where occupants receive supervised care, medical treatment, or are under security restraint.
Group M (Mercantile). Retail stores, markets, gas stations, and other spaces where goods are displayed and sold.
Group R (Residential). Subgroups R-1 through R-4. Buildings where people live or sleep.
Group S (Storage). Subgroups S-1 and S-2. Warehouses, self-storage, and parking garages, split by combustibility of stored goods.
Group U (Utility/Miscellaneous). Barns, carports, sheds, fences over six feet tall, and other structures that don’t fit elsewhere.

If a proposed use doesn’t match any listed category, the building official picks the group it most closely resembles based on fire safety and hazard level.² One useful edge case worth knowing: an assembly space with fewer than 50 occupants is classified as Group B rather than Group A, which spares it the heavier requirements that come with large gatherings.¹

Assembly Subgroups A-1 Through A-5

Assembly is the most divided group because the risks shift dramatically between fixed-seat theaters, open dining rooms, and outdoor stadiums:¹

A-1, performing arts with fixed seating. Movie theaters, concert halls, and live-performance theaters.
A-2, food and drink. Restaurants, bars, nightclubs, banquet halls, and casinos.
A-3, worship, recreation, and the catch-all. Churches, libraries, museums, courtrooms, bowling alleys, art galleries, gymnasiums without spectator seating, and community halls.
A-4, indoor spectator sports. Arenas, skating rinks, and indoor pools or tennis courts with spectator seating.
A-5, outdoor activities. Stadiums, grandstands, bleachers, and amusement park structures.

The A-3 versus A-4 line trips people up. A gymnasium where people play basketball is A-3. Add spectator bleachers around it and the same room becomes A-4, with different egress requirements driven by crowd density near the playing surface.

Residential Subgroups R-1 Through R-4

Residential covers any building where people sleep, as long as they don’t need supervised medical care or security restraint. The four subgroups turn on two questions: are occupants temporary or permanent, and do they need custodial care?¹

R-1, transient. Hotels, motels, and boarding houses for short-term guests. A property operating as long-term housing shifts to R-2.
R-2, permanent and multi-unit. Apartment buildings, dormitories, condominiums, live/work units, and vacation timeshares. The marker is more than two dwelling units with permanent residents.
R-3, small-scale residential. Single-family homes, duplexes, small boarding houses with 16 or fewer occupants, and care facilities serving five or fewer residents. Many R-3 buildings can follow the International Residential Code instead of the full IBC.
R-4, supervised residential care. Group homes, assisted living facilities, halfway houses, and similar settings for 6 to 16 full-time residents who receive custodial care. Above 16 residents, the building shifts to Institutional Group I-1.

That R-4 to I-1 threshold is one of the most consequential lines in the code. Going from 16 residents to 17 moves a facility out of residential rules and into institutional ones, with significantly more expensive fire protection and construction standards.

Institutional Subgroups I-1 Through I-4

Institutional classifications exist because occupants often can’t evacuate themselves. Requirements scale with how limited that ability is:²

I-1, supervised custodial care, 24-hour. Assisted living, group homes, and rehabilitation centers with more than 16 full-time residents who are generally capable of responding to an emergency with limited assistance.
I-2, medical care, 24-hour. Hospitals, nursing homes, psychiatric hospitals, and foster care facilities for more than five people who cannot preserve themselves in an emergency. I-2 further splits into Condition 1 (no surgery or emergency care) and Condition 2 (hospitals that provide both).
I-3, detention and correctional. Prisons, jails, detention centers, and reformatories where more than five occupants are under security restraint and cannot leave on their own.
I-4, day care. Facilities providing custodial care for more than five people of any age for fewer than 24 hours per day, covering both adult and child day care.

The I-2 Condition 1 versus Condition 2 split matters for healthcare developers. A long-term nursing home has different structural and fire suppression demands than a full-service hospital with surgery and an emergency department.¹

Factory, Storage, and High Hazard Subgroups

Factory and storage each split into a moderate-hazard and a low-hazard tier based on the combustibility of the materials involved:¹

F-1, moderate-hazard industrial. Woodworking, furniture manufacturing, printing, textile mills, dry cleaning, and food processing larger than 2,500 square feet.
F-2, low-hazard industrial. Fabrication of noncombustible materials like glass, metal products, brick, ceramics, and ice. The test is whether finishing or packing carries a significant fire hazard.
S-1, moderate-hazard storage. Warehouses storing combustibles like lumber, furniture, paper, leather, and clothing. Motor vehicle repair garages fall here as well.
S-2, low-hazard storage. Noncombustible items like metal parts, glass, and food in sealed containers. Private parking garages are S-2.

High Hazard is the classification most owners hope to avoid. Its five subgroups turn on the specific danger the materials present:¹

H-1, detonation hazard. Explosives, certain organic peroxides, and Class 4 oxidizers. These buildings face the most severe restrictions and often cannot share a structure with other occupancy groups at all.
H-2, deflagration hazard. Flammable gases, combustible dusts, pyrophoric materials, and flammable liquids in open or pressurized systems.
H-3, combustion hazard. Flammable liquids in closed containers at low pressure, consumer fireworks, flammable solids, and oxidizing gases.
H-4, health hazard. Corrosive, toxic, and highly toxic materials that threaten health rather than posing a fire or explosion risk.
H-5, semiconductor fabrication. A specialized classification for semiconductor plants handling a mix of hazardous materials under tightly controlled conditions.

How a Building Official Picks the Classification

Assigning the right label takes more than matching a building’s name to a list. Officials look at three interlocking factors.

The first is activities and materials. The type and quantity of combustibles drive the fire-risk picture. A warehouse full of paper and one full of steel shelving present different problems. When hazardous materials are involved, officials need a detailed inventory to see whether the building crosses the thresholds that trigger Group H.

The second is occupant load, calculated by dividing usable floor area by the occupant load factor assigned to each type of space in Table 1004.5.³ Common figures include:

Assembly with chairs only, no tables: 7 square feet per person
Assembly standing room: 5 square feet per person
Restaurant-style seating with tables and chairs: 15 square feet per person
Business offices: 150 square feet per person
Classrooms: 20 square feet per person
Industrial areas: 100 square feet per person
Retail: 60 square feet per person
Warehouses: 500 square feet per person

A 3,000-square-foot restaurant with tables and chairs has a calculated occupant load of 200 people (3,000 ÷ 15). That number drives how wide the exits need to be, how many exits are required, and whether the space is treated as an assembly occupancy at all. Underestimating the number means undersized exits, which is exactly the kind of error that costs lives in a fire.

The third factor is occupant capability. Buildings where people sleep need more robust alarm and detection because sleeping occupants respond slower. Buildings where occupants physically can’t leave on their own, like hospitals and jails, demand the most intensive fire protection because staff have to assist with evacuation.¹

Buildings With More Than One Use

A single structure often contains more than one occupancy. Retail downstairs with apartments above, or a warehouse attached to an office, are routine. The IBC gives owners two paths.

Nonseparated Occupancies

Under IBC Section 508.3, a building with multiple groups can run without fire-rated barriers between them, but the whole building then has to meet the most restrictive requirements of any group present.⁴ If one corner is Group A and the rest is Group B, the entire structure’s allowable height, area, and fire protection must satisfy Group A rules.

Separated Occupancies

Under Section 508.4, owners can divide groups with fire-rated walls and floor-ceiling assemblies. Each portion then follows its own classification. The required fire-resistance rating depends on which two groups sit on either side of the barrier and whether the building has sprinklers throughout.⁵ Table 508.4 runs from no separation required between similar groups up to four hours for the worst combinations, and some pairings, especially anything adjacent to an H-1 space, are flatly prohibited.

Choosing between the two methods is a cost calculation. Nonseparated means simpler interior construction and higher baseline standards everywhere. Separated means building fire barriers but using lighter construction and fewer sprinklers in the lower-risk portions. On a project with widely different risks under one roof, separated occupancy usually saves money.

Accessory Occupancies

Small secondary uses can skip the mixed-occupancy rules entirely. Under IBC Section 508.2, a space like an office inside a warehouse or a break room inside a factory qualifies as an accessory occupancy if it stays under 10 percent of the floor area of the story it occupies and under the building area limits in Table 503 for its occupancy type.⁶ An accessory space is still classified individually, but it follows the main occupancy’s construction type and height limits, and it needs no fire-rated separation.

What the Wrong Classification Costs

Running a building under the wrong classification is not a technicality. Building officials can revoke permits, deny future permits for the property, and declare the building a public nuisance until the violation is cured. In many jurisdictions, that authority follows the property to a new owner.

Insurance is the second problem. Commercial property policies typically require the building to comply with applicable codes. If a fire destroys a space being used in a way that doesn’t match its approved occupancy, the insurer may limit coverage for code-upgrade costs or deny portions of the claim tied to the unapproved use. Even when the claim gets paid, insurers often cancel or decline to renew coverage on a property with known code violations.

Then there is civil liability. A building classified Group B that is really being used as a Group A-2 restaurant almost certainly lacks adequate exits, suppression, and structural capacity for the real occupant load. If someone gets hurt, the gap between the label and the use becomes evidence in a negligence case.

One boundary worth flagging: converting a building from one occupancy group to another is governed by the change-of-occupancy provisions of the International Existing Building Code, which require a new certificate of occupancy and can trigger upgrades to sprinklers, alarms, structural capacity, and seismic resistance before the new use can begin.⁷ Picking the right classification at the start is cheaper than discovering mid-project that the building you bought needs to become something else on paper.

1
International Code Council. 2021 International Building Code – Chapter 3 Occupancy Classification and Use
2
International Code Council. 2024 International Building Code – Chapter 3 Occupancy Classification and Use
3
International Code Council. 2021 International Building Code – Chapter 10 Means of Egress
4
UpCodes. Nonseparated Occupancies
5
UpCodes. Separated Occupancies
6
International Code Council. IBC Interpretation 27-12
7
UpCodes. International Existing Building Code – Chapter 10 Change of Occupancy