Using Frontage to Increase Allowable Building Area

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.

A public way, such as a street that is at least 20 feet wide, counts towards the frontage increase.

​Using Frontage to Increase Allowable Building Area

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

​Performing the Frontage Calculation

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. Note that this method is valid through the 2018 IBC. Starting in 2021, the method has changed, which I will cover in a future update.

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

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

W = (L₁ x W₁ + L₂ x W₂...)/F

Where:
L_n and W_n are the length of wall segment and width of open space adjacent to that wall segment. Remember that W has to be at least 20 feet. If the width of the public way or open space is greater than 30 feet, you are required to use a maximum value of 30 feet for the calculation.

F is total building perimeter fronting a public way or open space with a width of 20 feet or more.

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

I_f = [F/P - 0.25] W/30

​ The frontage increase factor can then be used with IBC Equations 5-2 or 5-3 to determine the allowable area the building. In these equations, the frontage increase factor is multipled by the "NS" allowable area value from IBC Chapter 5.

Notice that with Equation 5-5, the maximum possible frontage increase factor is 0.75. In order to achieve this maximum value, the entire building perimeter has to front an open space or public way with width of 30 feet or more. If the width at any point is less than 30 feet, or if a portion of the building perimeter does not have any frontage, the frontage increase factor will be reduced.

You can also directly input the frontage increase factor into the allowable area calculators below:

Link: Separated Mixed Occupancy Area Calculator
Link: Nonseparated Mixed Occupancy Area Calculator

Conclusion

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.