A mixed occupancy building is any building that contains two or more occupancy types as defined by the IBC. The code requirements for mixed occupancy buildings are found in IBC Section 508, Mixed Use and Occupancy. Nearly every building contains multiple uses, but fewer contain multiple occupancy types (remember, there is a difference between use and occupancy). Assuming your building does have two or more of the occupancy types found in IBC Chapter 3, you have three available approaches:
Link: IBC Nonseparated Mixed Occupancy Height/Area Calculator
Link: IBC Separated Mixed Occupancy Height/Area Calcualtor
Note that these options are not exclusive, as the IBC specifically states that you can use a combination of these approaches in your building design. It’s common to use both the accessory and nonseparated approaches together to avoid providing a rated separation between occupancies.
Building designers are also free to use any of these approaches, as there is no requirement stating one approach or the other must be used. I have talked with many building owners who believe they have to provide a rated separation between occupancy types (the separated occupancy approach). The code allows any of the mixed occupancy approaches to be used, so the only situation where a separated approach is required is when the design cannot comply with the other approaches.
An accessory occupancy approach can be used when one of the occupancies in the building is ancillary to the main occupancy. A common example of an accessory occupancy approach is a conference room located within an office building. Another is an exercise room provided for residents of an apartment building.
In order to be considered an accessory occupancy, the aggregate area of accessory spaces must be less than 10% of the floor area of the story in which its located. The area of the accessory occupancy must also be less than the allowable area for that occupancy type found in IBC Table 506.2, using the nonsprinklered “NS” value.
The main advantage of using the accessory occupancy approach is that the allowable height, area and number of stories of the building is based on the main occupancy of the building. Using the example above, an office building that classifies the conference room as an accessory occupancy would only need to comply with the height/area requirements for a Group B occupancy, not Group A.
Accessory occupancies do not require a rated separation from the main occupancy, unless there are Group H, I-1, R-2, or R-3 occupancies present, in which case the special separation requirements forum in IBC Chapter 4 still apply.
By far, the most common misunderstanding of the accessory occupancy approach is that other code requirements beyond the allowable height/area can be ignored. Outside of the allowable height and area, all of the code requirements for the accessory occupancy still apply to that portion of the building. Continuing with the example above, if the conference room is a Group A-3 occupancy, even if it is considered accessory, all of the code requirements for Group A occupancies still apply to the conference room. So, the travel distance, common path of egress travel, and dead-end corridor lengths of this conference room would all need to comply with the requirements for Group A occupancies.
Any fire protection requirements found in IBC Chapter 9 would also apply to the accessory occupancy. If IBC Chapter 903 requires a sprinkler system for the accessory occupancy, that would still apply, even if a sprinkler system was not required for the main occupancy.
In a nonseparated occupancy approach, there is no requirement for a fire rated separation between adjacent occupancies.
The key limitation to the nonseparated occupancy approach is the allowable height, area and number of stories are limited by the most restrictive requirements of the occupancies in the building. So, if one of the occupancies in the building is limited to two stories per IBC Table 504.4, the entire building is limited to two stories, even if the other occupancy types are permitted additional stories. Check out our calcualtor that quickly shows you the height/area requirements for this approach.
Similarly, the most restrictive fire protection requirements for the occupancies involved must be applied throughout the entire building (or nonseparated area). So, if a building has two occupancy types and one of them requires a voice fire alarm system, the system would need to be provided for the entire building.
If the building falls within the allowable height, area and number of stories for all the occupancy types involved, and you can meet the fire protection requirements of the most restrictive occupancy, a nonseparated approach is the easiest from a design standpoint. There is no requirement to provide a rated separation between nonseparated occupancies, unless there are Group H, I-1, R-2, or R-3 occupancies present, in which case the special separation requirements forum in IBC Chapter 4 still apply.
Outside of the Chapter 9 fire protection requirements and height/area requirements, other code requirements are applied individually to each occupancy type in the building.
With a separated mixed occupancy approach, a fire rated separation is typically provided between adjacent occupancy types per IBC Table 508.4.
The main advantage to a separated occupancy approach is that the allowable height, area and number of stories for the building are applied individually to each occupancy. Going back to the previous example of a building where an occupancy is limited to two stories in height, the building could be taller, with other occupancies on the upper stories, as long as there is a rated separation between the second story and areas above.
The allowable area is based on the sum of the ratios of the actual floor area for each occupancy divided buy the allowable floor area of that occupancy. The sum of these ratios cannot exceed one for any given floor and three for the entire building (assuming an NFPA 13 sprinkler system). This approach typically allows for a larger total building area than the other mixed occupancy approaches (check out the calculator for this if you need further assistance).
Other code requirements, such as egress or fire protection requirements are applied individually to each occupancy. If the separated approach is being taken in conjunction with one of the other approaches (accessory or nonseparated), the most restrictive fire protection requirements from Chapter 9 are applied to the total nonseparated occupancy area.
A common point of confusion is that in some cases, the separated occupancy approach can be taken without providing a rated separation between occupancies. IBC Table 508.4 shows that occupancy types of similar hazard level have a required separation level of “N,” meaning there is no separation requirement. For example, a building containing both Group B and M occupancies could take the separated occupancy approach but not actually provide any rated separation between the occupancies. This is a little confusing to understand, but essentially, the code views certain occupancies as similar hazard levels and allows these areas to be continuous to one another without a rated separation.
The IBC offers three approaches to mixed occupancy buildings: accessory, nonseparated and separated. One or more of these approaches must be used in any building containing two or more occupancies. The IBC does not require any of these approaches specifically, rather a designer can choose from any of the three approaches that best suit the design goals.