Mixed Used Buildings - Why You Probably Have Been Saying It Wrong All Along

​In the AEC world, the terms “mixed use” and “mixed use building” are commonplace. Developers refer to new buildings that contain both office and retail spaces as mixed use projects. Architects and Engineers refer to a floor containing multiple occupancy types as mixed use. You’ll even hear AHJs drop the term mixed use in plan review meetings on occasion.

Colloquially, “mixed use” is meant to refer to a building or space that contains multiple occupancy types. But did you know that other than parking garages, the building code has essentially no requirements pertaining to a mixed use building? In fact, if you search for “mixed use” in the International Building Code (IBC), you won’t find any requirements related to building height, building area, construction type, fire-rated construction or means of egress. *

So why is “mixed use” a common term in the AEC world but not addressed in the codes? Long story short: there is a difference between use and occupancy in the IBC and people frequently confuse the two. We’ll do a full analysis of the differences between use and occupancy in a later article, but here is a simple way to understand the difference.

This is both a mixed used and mixed occupancy building (image courtesy of Brett VA).

Mixed Use Vs. Mixed Occupancy

The use of a space is a description of how the space will actually be used. This could be a broad, general description such as “office” or “conference room”, or it could be more specific, such as “visiting team locker room and shower.” The use of a space is helpful in determining (1) the occupancy classification and (2) the appropriate occupant load for a space.
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On the other hand, the occupancy classification of a space is 1 of 10 categories (plus sub categories) in Chapter 3 of the IBC. These occupancies are:

• Assembly – Groups (A-1, A-2, A-3, A-4, A-5)
• Business – Group B
• Educational – Group E
• Factory – Group F
• High Hazard – Groups H-1, H-2, H-3
• Institutional – Groups I-1, I-2 (both with Conditions 1 and 2)
• Mercantile – Group M
• Residential – Groups R-1, R-2, R-3, R-4
• Storage – Groups S-1, S-2
• Utility and Miscellaneous – Group U

While occupancy classification is limited to this list, there are essentially an unlimited number of potential uses for a building or space. The use of a space is certainly a factor in determining the occupancy classification, but it is not the only one. Chapter 3 of the IBC has very specific definitions for each occupancy group, and these definitions can result in the occupancy classification of a building or space varying from what might be expected based on the use.

For example, most offices and places of business have conference rooms, where multiple people come together for meetings, presentations or collaboration. The use of such spaces might lead you to think that the conference room is an assembly occupancy. But per IBC 303.1, if your office has a conference room with 49 people, the conference room is considered a business occupancy. There are many similar examples where the use of a space does not necessarily align with the occupancy, so always refer to the definitions in Chapter 3 when determining the occupancy classification.

Now getting back to the original discussion, “mixed use buildings” do not have specific requirements because nearly every building contains multiple uses. Does your office suite have a storage area? You have at least two uses there. Does your retail store have an office in the back? Multiple uses. Does your apartment building contain an amenity space? You get the point.

So when people use the term “mixed-use,” it doesn’t mean much from a code standpoint, and practically, almost all buildings contain multiple uses. But what most folks intend to convey by this term is that the building has at least two distinct occupancy types, which the IBC would consider “mixed occupancy.” And unlike “mixed-use,” there are numerous requirements for “mixed occupancy buildings” in the IBC. In fact, IBC Chapter 5 has sections for allowable height, allowable area, allowable number of stories and required separation of occupancy, all of which are impacted when a building is mixed occupancy.

Conclusion

Conversationally, “mixed use building” is generally understood to mean a building containing multiple occupancy types. Architects, engineers, contractors and code officials all use this term and it typically does not create a misunderstanding. But as far as the code language goes, mixed use doesn’t mean anything for the requirements for a building. Mixed occupancy, on the other hand, carries many code requirements that must be understood for a code-compliant building design.

*While IBC 508 is entitled "Mixed Use and Occupancy,' it does not actually have specific requirements for mixed use buildings, only mixed occupancy buildings.