The IBC offers 3 approaches to mixed occupancy buildings: accessory, nonseparated and separated. Any building with two or more occupancy types must choose one or more of these approaches for compliance. In this post, we will cover all three of these approaches and the specific requirements related to each.

In a nonseparated occupancy approach, there is no requirement for a fire-rated separation between adjacent occupancies. However, the key requirement in a nonseparated occupancy approach is that the most restrictive requirements for the allowable height, area and number of stories, as well as the most restrictive fire protection requirements must be applied throughout the entire building. For example if your building has two occupancy types and one of them requires a voice fire alarm system, a nonseparated occupancy approach would mean that you need to provide a voice fire alarm system throughout the building.

If your building falls within the allowable height and area for the occupancy types involved and you can meet the fire protection requirements of the most restrictive occupancy, a nonseparated mixed occupancy approach is the easiest from a design standpoint because there is no additional requirements for rated separations.

The separated mixed occupancy approach is typically provided when the non-separated approach is not an option. With a separated mixed occupancy approach, a fire-rated separation is typically provided between each occupancy type. IBC Table 508.4 is referenced for the specific rating requirement, though a rated separation may not be required between occupancies of a similar hazard level.

The allowable height and number of stories is applied for each individual occupancy type. For example, if your construction type limits a Group A occupancy to 4 stories, your building could be taller as long as the Group A occupancy is not located above the fourth story.

The allowable area is based on the sum of the ratios of the actual floor area for each occupancy divided by the allowable floor area of that occupancy. The sum of these ratios cannot exceed one for any given story. Requirements for egress and fire protection systems are applied invidivualy to each separated occupancy in the building.

The final option is an accessory occupancy approach. This method is applicable where there is a small portion of a building or space that is subsidiary to the main occupancy. When that smaller space is 10% or less of the total area of a story, an accessory occupancy approach can be used.

There is no required separation between the accessory and main occupancy, and the building height, area and number of stories are based on the main occupancy, not the accessory occupancy. If the building contains a Group H, I or R occupancy, those separation requirements found in Chapter 4 would still apply.

Within the accessory occupancy, other code requirements such as egress and fire protection systems are based on the accessory occupancy classification, not the main occupancy. This is crucial point that confuses many people. Essentially, the accessory occupancy designation allows you to ignore the height, area and number of stories requirements for the accessory occupancy, but all of the other code requirements for the accessory occupancy still apply.

For example, a large conference room located within an office building could be considered an accessory occupancy, so the limitations for height, area and number of stories for Group A-3 occupancies would not apply. But all of the other code requirements for Group A-3, including means of egress and fire protection requirements would still apply!

One final option - if you are able to separate multiple occupancies using a fire wall, you create two separate buildings from a code application standpoint and you can consider each building indvidually. A fire wall is the most robust separation option available though, so its likely to be the most disruptive from a design standpoint.

In summary, there are three approaches to mixed occupancy buildings: non separated, separated and accessory. All buildings with multiple occupancies must use one or more of these approaches. If your building contains three mor more occupancy types, it's possible to use a combination of the approaches, such as designating an accessory occupancy and then taking a nonseparated approach for the remaining occupancies.

If you have designed a building of Type II, III, IV or V construction, you probably considered the allowable building area, which depends on the construction type, occupancy classifications, presence of fire sprinkler systems, and a few other criteria.
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One common way to increase the allowable area for a building is to use a frontage increase. The frontage increase provision in the IBC, found in Section 506.3, recognizes the safety benefits of having open space directly adjacent to a building. When a fire department or other emergency responders arrive to a site, there is an increased level of effectiveness when those personnel can access a large portion of the building exterior.

There is no requirement for a building to use the frontage increase provisions of IBC Section 506.3, but when used, the following requirements apply:

• A minimum of 25% of the building perimeter must be adjacent to a public way or open space
• To count as frontage, the open space must be either on the same lot or a dedicated public space. Open space on an adjacent lot does not count.
• The width of the open space must be at least 20 feet, measured at right angles from the building exterior wall to: (1) the closest interior lot line, (2) the entire width of the public way, or (3) the exterior face of a building on the same property.
• The open space must be accessed from a street or fire lane

The frontage calculation can be performed by hand, but I recommend the building frontage calculator. If you want to do the calculation on your own, here are the steps:

1. Determine the weighted average of the width of open space (W). 

This value, W, is determined by the following equation:

2. Determine the total building perimeter, P.
3. Solve IBC Equation 5-5 to determine the frontage increase factor. 

Using the frontage increase provisions of IBC 506.3 is a common way to increase the allowable area of a building. There is no requirement to use these provisions, but when you do, the requirement described above do apply.
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When working with architects, I frequently see code summary sheets with a frontage calculation for Type I buildings that have an unlimited allowable area. Since there is no requirement to take a frontage increase, I typically remind architects they don’t need to perform the frontage calculations if the building already has an unlimited allowable area. This saves time and also prevents possible permit review comments on a code provision that wasn’t needed in the first place.

As more and more states move to adopt the 2018 IBC, it's important to know about a few code updates that impact the design of occupied roof spaces. And if you're jurisdiction is on the ball and already adopted the 2021 IBC, there are a few additional items that apply to you.

In the 2015 IBC and prior editions, the code remained silent in regards to occupied roof decks and requirements for allowable height and area.  Under the 2018 IBC, there are now specific provisions addressing this issue.

First, IBC 302.1 has been updated with specific requirements for classifying an occupied roof space:

This section limits the occupancy on an occupied roof to those allowed on the story immediately below the roof. So if your building construction type allows an assembly occupancy on the top story, you are also permitted an assembly occupancy on the roof. The code does offer two major exceptions:

1. If your building if fully sprinkler-protected (NFPA or 13R) and you have fire alarm notification on the occupied roof, you are permitted to have any occupancy on the occupied roof.
2. If your building is Type I or II construction, you are permitted to have assembly occupancies on the roof of parking garages.

Take the image below for an example, a 4 story building consisting of Type IIIA construction. If the building is fully sprinkler protected in accordance with NFPA 13, a Group A-3 occupancy would per permitted on Level 4. As long as occupant notification is provided on the roof, the 2018 IBC now explicitly allows a Group A-3 occupancy on the roof.

The 2021 IBC updates Section 503.1.4 to explicitly clarify that an occupied roof should not be included when determining building height or number of stories per IBC 504. Language was also added to clarify that this only applies when penthouse and other rooftop structures, when present, comply with IBC 1511. This further supports the notion described above that an occupied roof should not by itself trigger a classification as a high-rise building.

Additionally, the 2021 IBC clarifies that the Exception 1 to 503.1.4 for occupant notification would only require a voice fire alarm system on the roof if the system is required elsewhere in the building. In other words, if you aren't required to have a voice fire alarm in the building, you don't have to provide it on the roof in order to use Exception 1.

Finally, the 2021 IBC updates Section 1511.2.2, Use Limitations for Penthouses, to specifically allow "ancillary spaces used to access elevators and stairways" to be considered part of a penthouse. This means that a stair or elevator tower to the roof will not force an occupied roof to be classified as a story, even though those elements are taller than 48" from the roof surface.

In the past few months, I've had several clients ask me questions regarding grade plane calculations - most often related to determining whether a particular story counts as a story above grade plane or as a basement. Here is quick rundown of how to do the calculation and the implications.

Chapter 2 of the IBC defines Grade Plane (often referred to synonymously as Average Grade Plane) as:

A reference plane representing the average of finished ground level adjoining the building at exterior walls.

So if you have a completely flat site, the elevation where the finished ground level adjoins the building exterior walls is your grade plane. If your site is sloped, you need to take several elevations where the finished ground level adjoins the building and average them to determine the grade plane.

The most common question I receive on this: how many elevation points do you need to account for this in average?

First, the IBC does not give any specific requirements or instructions to answer this question. Your local jurisdiction could potentially have some guidance on how they want to see the calculation performed, but in my experience, most do not.

Typically, I advise clients to base the number of elevation points on the slope of the site. If you have a relatively flat site, a small number of elevation points will give you an accurate grade plane elevation. You may only need one elevation point per face of the building. On the other hand, if you have an irregular, highly-sloped site, you will need many elevation points. I once worked on a building that was built into the side of a steep hill - there we measured the elevation every 10 feet in some areas. If in doubt, the greater number of elevation points in your calculation will result in a more accurate measurement.

In the simple example above, the grade plane measurement is just the average of the elevations at the end of each wall segment (e.g. for the east wall, (280+270)/2 = 275'). However, with a more severe grade, particularly where the slope is irregular, you will need to take additional measurements, as shown in the example below.

Why does the grade plane calculation matter? On my projects, this always comes up when a client is trying to determine whether a particular story is actually a story above grade plane or a basement.

Chapter 2 of the IBC defines "Story Above Grade Plane" as:

Any story having its finished floor surface entirely above grade plane, or in which the finished surface of the floor next above is:

1. More than 6 feet (1829 mm) above grade plane; or
2. More than 12 feet (3658 mm) above the finished ground level at any point.

This definition has caused some confusion in the past, so here is a step by step breakdown:

1. Is the floor surface of the story in question located entirely above the grade plane elevation?
2. Is the floor surface of the floor above the story in question located more than 6 feet above the grade plane elevation?
3. Is the floor surface of the floor above the story in question located more than 12 feet above any of the grade measurements at any point along the building exterior walls?

If you answered yes to any of the three questions above, the floor in question is considered a story above grade plane.

There are numerous code requirements that differ between a story and a basement, but the most common one is complying with the allowable number of stories based on occupancy and construction type. I had a project this year where the floor surface of the level above was 5 feet, 10 inches above the grade plane elevation, just barely passing as a basement. 2+ inches more and the building would haven been one story taller, resulting in requirements for a more robust construction type, higher shaft ratings and substantial additional cost.

If your situation is that close, I suggest including a plan in the permit package that clearly shows the grade elevations around the building and shows your grade plane calculation. This makes it easy for plan reviewers to follow your logic and hopefully agree with your approach (remember, there are no specific IBC instructions or requirements for how to do the calculation).

Have you had a project that required a detailed grade plane calculation? Have you every had a plan reviewer or code official disagree with your calculation? Let me know in the comment box!

October 2020 Update: I am moving all calculators to the Tools menu at the top right of the screen. I have also added in a slider bar to allow for varying frontage amounts (see the new tools page for this function).

​After several weeks of working on this, I'm excited to release a beta version of the 2015 IBC Allowable Height, Area and Number of Stories Calculator. Take a look and let me know what you think!

A few notes:

• Calculator is for single occupancy or non-separated mixed occupancy buildings. Separated mixed-occupancy functionality is coming later.
• There are some error notes that I've programmed in (for example, using an NFPA 13R sprinkler system in a non-residential occupancy), but the calculator is not foolproof. Some background code knowledge is needed.
• Allowable area results are displayed for no frontage increase and full frontage increase. You'll need to do your own exact frontage calculation if needed.

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.