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.