Use and Occupancy Classification refers to how a building or portion of a building is used or intended to be used. The IBC divides buildings into ten major occupancy groups (with multiple subgroups), and each has specific implications for safety, code compliance, and design.

You can find the full list and definitions of each Occupant Group in the 2021 IBC Chapter 3 Occupancy Classification and Use. Those groups are then used in various other sections of the code to determine things like egress distance, fire separation, allowable building area, allowable building height and more.

In this post I'll cover the various groups, how they relate to other code sections (like Fire Separation) and some related terms, like Accessory Occupancy and Incidental Use.

But first, a sample problem! Answer all the way at the bottom, knowledge to solve this in the content of this post.

Use Group ARE 5.0 Sample Problem

QUESTION

A new three-story building includes a first-floor café (2,000 sf), a second-floor architecture office (2,000 sf) and a third-floor adult education center with classrooms for 25 students. The cafe has a 175 sf storage room for food, cardboard boxes and various types of paper bags for to-go orders. What occupancy groups are present? Would the answer change if the storage room was 250 sf?

(scroll down for answer)

Use and Occupancy PRESENTATION

I recently did a presentation on this topic, and Occupant Load for 40 Minutes of Competence, a weekly free seminar hosted by Amber Book.

You can also watch the full presentation right here. Check out the YouTube description for a link to download the slides.

2021 IBC Occupancy Groups

The following is a quick summary of 2021 IBC Chapter 3. I did NOT have any of this stuff memorized for my exams. I did have a general understanding of the different groups, like A vs M vs S, and I did know how to find the information I needed in Chapter 3.

Assembly Group A

“…for the gathering of persons for purposes such as civic, social or religious functions; recreation, food or drink consumption or awaiting transportation.” (2021 IBC 303.1)

Assembly Occupancies are places where people gather; theaters, restaurants and clubs, libraries, museums, arenas, etc. See Section 303 of the 2021 IBC.

There are five Assembly subgroups:

Assembly Group A-1
Performing arts; movie theaters, concert halls, theaters

Assembly Group A-2
Intended for food and drink consumption; casinos, nightclubs, restaurants, taverns

Assembly Group A-3
Worship, recreation, amusement or other; art galleries, bowling alleys, courtrooms, libraries, museums

Assembly Group A-4
Viewing (or doing?) indoor sports; arenas, skating rinks, swimming pools, tennis courts

Assembly Group A-1
Participating in, or viewing, outdoor activities; amusement park structures, bleachers, stadiums

What to ACTUALLY know about Assembly Group A

• • That Assembly info can be found in 303 (or just in Chapter 3)
  • That there are different types of assemblies based generally on what happens there
  • That ALL the assemblies are treated the same for fire separation (see 508.4). This means that if the project (or the ARE question) requires a determination of fire separation between a group A area of the building and some other group, it doesn't matter if it's A-1 or A-5…the fire separation table just lists ‘A'
  • maybe be able to quickly tell the difference between A1, A2 & A3 (I couldn’t)

Business Group B

“…for office, professional or service-type transactions, including storage of records and accounts.” (2021 IBC 304.1)

Business occupancies are places where professional services are offered. This is not stores, which are covered in Group M, but things like banks, architecture offices, Macro-Data Refinement and college classrooms (but not lecture halls, which are A-3!)

There are no Business sub groups, and the code considers Group B to be relatively low risk.

Educational Group E

“…by six or more persons at any one time for educational purposes through the 12th grade.” (2021 IBC 305.1)

All of Education (up to 12th grade) is classified under ‘E’ Occupancy. Variable risk to life safety due to children needing more protection than adults. Note that Educational ends at high school. College classrooms are usually Business or Assembly.

Things like cafeterias, gymnasiums and libraries may be classified as ‘A’ resulting in a mixed occupancy building – OR – may be classified as ‘E’ if used solely for school purposes*

* this is the kind of thing I would NOT memorize and I doubt the ARE will try to trick you or test you on that level of code analysis.

Mercantile Group M

“…for the display and sale of merchandise, and involves stocks of goods, wares or merchandise incidental to such purposes and where the public has access” (2021 IBC 309.1)

This is what we usually call RETAIL. It includes stores and places where things are for sale.

The 2021 IBC considers Group M to pose a higher risk to life safety than Group B because:

• • possible hazard of items stored there
  • possible larger crowds (think department store vs insurance office)
  • less familiarity with the people there (people shopping at a store once vs people who work in the same office every day)

Residential Group R

“…for the display and sfor sleeping purposes when not classified as an Institutional Group I or when not regulated by the International Residential Code” (2021 IBC 310.1)

Residential is where people SLEEP and sometimes LIVE. There are four Residential subgroups, generally classified by how many sleeping places there are and how long people stay there. Residential includes things like hotels, motels, dorms and apartment buildings.

The four Residential subgroups are:

R-1
Transient; hotels, motels, boarding houses

R-2
Permanent, more than two dwelling units; apartments & dorms

R-3
Permanent, two or fewer dwelling units, not otherwise classified

R-4
Between 5 and 16 people, not including staff, who reside and are supervised 24 hours a day; assisted living, group homes, alcohol and drug centers

What to ACTUALLY know about Residential Group R

• • That Residential Group R can be found in section 310
  • That there are different types of Residential based generally on how big or how long people stay there
  • That ALL the R subgroups are treated the same for separation. Just like Group A above. Some sections of the code may use R-1 or R-2 etc to set requirements, but not Table 508.4. For fire separation purposes it's just Group R.

I think you're a less likely to encounter the remaining three groups on the exams.

Factory Industrial Group F

“for assembling, disassembling, fabricating, finishing, manufacturing, packaging, repair or processing operations that are not classified as a Group H hazardous or Group S storage occupancy.” (2021 IBC 306.1)

Factory Group is split between Moderate Hazard F-1 and Low Hazard F-2. See section 306.2 and 306.3 for more.

High-Hazard Group H

“involves the manufacturing, processing, generation or storage of materials that constitute a physical or health hazard…” (2021 IBC 307.1)

The word HAZARD is in the name of this group, so you know it's a dangerous place to be. Group H will tell you things like you can only store up to 45 gallons of Cryogenic Flammable or 50 lbs of Division 1.4 Explosives before needing to classify as Group H. I have no idea what any of that stuff is.

Institutional Group I

“in which care or supervision is provided to persons who are or are incapable of self-preservation without physical assistance or in which persons are detained for penal or correctional purposes or in which the liberty of the occupants is restricted” (2021 IBC 308.1)

The institutional group is for people not capable, or not allowed, to care for themselves. This includes Hospitals, detox centers, jails and assisted living facilities. Assisted living facilities are ALSO included in Residential, which we covered earlier. I bring this up as a reason to NOT remember this stuff. I really don't think you'll need to determine whether an assisted living facility is I or R-4. Maybe in real life, definitely not on the exam.

INCIDENTAL USE & ACCESSORY OCCUPANCY

Incidental Use and Accessory Occupancies occur, or are allowed, when a small portion of the building area is used for a purpose that's different than the primary Occupancy of the building. They are similar, but not quite the same. Incidental Use is considered more hazardous than Accessory Occupancies.

Incidental Uses (Section 509)

“…Incidental uses are ancillary functions associated with a given occupancy that generally pose a greater level of risk to that occupancy” (2021 IBC 509.1)

Ancillary means secondary, auxiliary or assisting. Incidental Uses are support spaces, things like furnace or machinery rooms, large laundry rooms or storage. This is not stuff like cafes, kiosks or more public spaces. There is a very specific and limited number of uses that can be classified as Incidental Use. See Table 509.1

Incidental Use Key Points

• • Incidental Uses spaces do NOT have their own Occupancy Classification. That means if the Incidental Use space is in an R-1 building, the Incidental Use space is also R-1, even if it would have been something else if it was a larger portion of the building.
  • Incidental Use spaces DO have their own Occupant Load. More on that in a future post, but Occupant Load is a calculation of how many people could be in a space, based on the size and use of a space. So for Incidental Use you have to calculate that space separately.
  • Incidental Use spaces MUST be provided with fire separation, per Table 509.1
  • Incidental Use spaces are limited to 10% of the floor area on the floor on which they are located. Same for Accessory Occupancy

Accessory Occupancy (508.2)

“…those occupancies that are ancillary to the main occupancy of the building or portion thereof” (2021 IBC 508.2)

Accessory Occupancies are different from the principal occupancy, but support the principal occupancy and wouldn't exist apart from the principal occupancy. I always think of this like a coffee kiosk with a seating area in a mall or other large space. The kiosk wouldn't be it's own store and wouldn't exist if not for the mall. But it's clearly different than the rest of the shopping areas.

Accessory Occupancy Key Points

• Accessory Occupancies DO have their own Occupancy Classification. If the primary use is A-1 then the Accessory Occupancy area might be M.

• Accessory Occupancies DO have their own Occupant Load. Just like Incidental Use, you need to calculate this area separately.

• Accessory Occupancies DO NOT require fire separation.

• Accessory Occupancies are limited to 10% of the floor area on which they are located, OR, the non-sprinklered tabular area from Table 506.2

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Use Group ARE 5.0 Sample Problem answer

ANSWER

A-2 (cafe)

B (architecture office)

B (adult education center)

S-1 (storage)

The answer does NOT change if the size of the storage room increases. As an Accessory Occupancy it already had it's own Occupancy classification. At 250sf it would exceed the limits of an Accessory Use, which may require fire separation if the building is designed to make use of separated occupancies.