Calculating Allowable Heights, Areas, and Story Heights

The allowable maximum area and height envelope of a building is influenced and regulated by many things. These can include client needs, desired architectural features, green building and daylighting concerns, and zoning regulations. One aspect that may be forgotten at early stages and unknown by a client are the building code regulations governing such items.

The story can happen such as this: the client asks for a design of a particular building height and the planning department restricts the height to another height. Much effort is placed into the design of a building to meet both the regulatory and client needs, then the client informs the architect at the worst time that their investor requires additional equity in the property, so an additional story to secure the financing is needed. The extra story can inevitably push the envelope higher than allowed by Chapter 5 for a particular type of construction. This could create an increase in estimated construction cost significantly if the design is moved from combustible construction to non-combustible construction.

To illustrate these topics we will use the Wyoming Building Code as it shares a lot of commonality with other jurisdiction’s codes. Make sure to reference your jurisdiction’s code to adapt to your project.

Here we will look into each of the three components of the building envelope:

1. Building area.
2. Allowable building story height.
3. Allowable height in feet.

Allowable Building Areas

To determine the allowable building area in accordance with Section 506 of the building code. Several factors are part of the determination of allowable areas:

• Occupancy groups as defined by Chapter 3,
• The presence or not of an automatic sprinkler system types as defined by Section 903.3, regulated by NFPA 13,
• Factors for area increases determined by the widths of public ways and open spaces around the building, and
• The type of construction of the building.

First, determine the occupancy groups of the spaces involved in the building. Careful reading of Chapter 3 will help determine various types of business, assembly, educational, institutional or industrial uses.Second, establish whether or not the building has an automatic sprinkler system, and what type. Most likely, new buildings of size will have a sprinkler system compliant with NFPA 13. Existing buildings may or may not have a sprinkler system, and previous permits for the building are the most reliable place to check for determining the sprinkler system of such a building.Third, establish the type of construction of the building (denoted with a roman-numeral and either A or B type). The type of construction will be one of the following types established in the building codes adopted as of the writing of this article. (Note, additional building types will soon be available for mass timber buildings).

• Type IA and IB - Non-combustible construction (i.e. steel or concrete) with high to moderate levels of fire protection (typically high-rise and specialty buildings such as jails in California)
• Type IIA and IIB - Non-combustible construction with either low (“one-hour”) or no fire-resistant construction.
• Type IIIA and IIIB - Non-combustible exterior walls with a combustible (i.e. wood framed) roof with either low or no fire-resistant construction.
• Type IV - heavy timber (and soon to include subtypes for mass-timber construction).
• Type VA and VB - constructed of any allowable material, including wood construction, with low or no fire-resistant construction.

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Area Increase for Frontage on a Public Way or Open Space

If the building fronts on a public way, an increase in the area due to this frontage on a public way can be used to increase the allowable area of the building. The area increase factor is defined in Section 506.3.3 is as follows:

where:

• ‍I_f = Area factor increase due to frontage.‍
• F = Building perimeter that fronts on a public way or open space having minimum distance of 20 feet (6096 mm).‍
• P = Perimeter of entire building (feet).‍
• W = Width of public way or open space (feet) in accordance with Section 506.3.2.

If in addition to the public way, the building also has yards or open space adjacent to the public way, these open space areas may be included as part of the frontage of the building. To qualify, the distance for the frontage increase must be at least 20 feet and measured at right angles to the building. The open space must be clear from the ground to the sky. Since this space is generally considered an extension of the public way for the purpose of fire department access, it is important that any features found in this area would only be similar items that may be found in a public way, such as light poles, low landscaping, parking, and the like. This would not generally include storage containers, pallets, permanent mechanical equipment, area under projections, awnings or trellises. If an item would prevent access by a fire truck and could not be moved, it likely disqualifies at least part of the area as count toward a frontage increase.

The width of the frontage area is measured to one of the following:

1. The closest interior lot line.
2. The entire width of a street, alley or public way.
3. The exterior face of an adjacent building on the same property.

To qualify for an area factor increase based on frontage, at least 25 percent of its perimeter must front on a public way or open space that is on the same lot or dedicated for public use and shall be accessed from a street or approved fire lane.

Building Composition and Configuration

The allowable area for a particular occupancy is generally determined by the equation

• Aa = allowable area (in square feet)
• At = tabular area form table 506.2 based on the sprinkler type (NS, S13D, S13R, or SM as applicable)
• NS = tabular area for the non-sprinklered condition
• If = area increase factor based on frontage

The allowable area for the building will be calculated slightly differently depending on a number of factors, including:

1. Single or Multistory
2. Single or Multiple Occupancy
3. Fire-separated or Non-separated

From table 506.2, one can see that for an A-3 occupancy group, sprinklers allow a four-fold increase in the allowable area for a single story condition, and a three-fold increase in the allowable area for a multistory condition from a non-sprinklered condition. However, for some high-hazard occupancy groups, areas are not increased for the presence of an NFPA 13 automatic sprinkler system.

The ratio of the actual area per floor to the allowable area (Aa) should be equal to or less than 1.0. if the occupancy is mixed with other occupancies (i.e. Mixed Use and Occupancy), then the allowable area depends on whether or not fire barriers are provided between occupancy groups, or if there is no fire separation between occupancy groups. For a fire-separated occupancy condition, the sum of the ratios of the actual area to the allowable area should not exceed 1.0. For a non-separated occupancy, the actual area cannot exceed the most conservative allowable area for all the occupancy groups considered.

In this case, a two story mixed occupancy building with a Group B (3,000 sq. ft) and H-3 (1,000 sq. ft) occupancy on the first floor and a Group B (4,000 sq. ft.) occupancy on the second floor, with the area increase for frontage = 0.283

First Floor:

A_a (B) = A_t+(NS*I_f) = 27,000 + (9,000 * 0.283) = 29,547 sq. ft.A_a (H-3) = 5,000 + (5,000 * 0.283) = 6,415 sq. ft.Area(H-3) / A_a(H-3) + Area(B) / a_a(b) = 3,000 / 29,547 + 1,000 / 6,415 = 0.11 + 0.16 = 0.26 < 1.0 → OK

Second Floor:

Area(B) / A_a(B) = 4,000 / 29,547 = 0.14 → OK

Allowable Building heights: Building Height in Feet

Table 504.3 shows the allowable height in feet for a building based on occupancy group. The use of an NFPA 13 automatic sprinkler system will generally provide a height increase of 20 feet from the non-sprinklered condition. Sprinklers are also mandated for certain occupancies, such as new Group H or R occupancies.

Allowable building heights, measured in feet, are not allowed to be higher than shown in Table 504.3. For mixed occupancy groups, the occupancy group in question may not be higher than the allowable height for that group. Building height is measured from grade plane to the average height of the highest roof surface.

[BG] HEIGHT, BUILDING. The vertical distance from grade plane to the average height of the highest roof surface.

To properly measure the building height, careful consideration should be given to the values of the grade plane and the average roof height, particularly if sloping sites or sloping roof planes are present. For example, a multistory building with a Group B occupancy and a Group H-3 occupancy have differing allowable building heights. This would mean that the roof surface above the B occupancy could not exceed 60 feet, and the roof surface (or floor surface of the floor) above the H-3 occupancy could not exceed 40 feet for a sprinklered building.To properly measure the building height, careful consideration should be given to the values of the grade plane and the average roof height, particularly if sloping sites or sloping roof planes are present. For example, a multistory building with a Group B occupancy and a Group H-3 occupancy have differing allowable building heights. This would mean that the roof surface above the B occupancy could not exceed 60 feet, and the roof surface (or floor surface of the floor) above the H-3 occupancy could not exceed 40 feet for a sprinklered building.

Allowable Building heights: Building Height in Feet

Table 504.3 shows the allowable height in feet for a building based on occupancy group. The use of an NFPA 13 automatic sprinkler system will generally provide a height increase of 20 feet from the non-sprinklered condition. Sprinklers are also mandated for certain occupancies, such as new Group H or R occupancies.

Allowable building heights, measured in feet, are not allowed to be higher than shown in Table 504.3. For mixed occupancy groups, the occupancy group in question may not be higher than the allowable height for that group. Building height is measured from grade plane to the average height of the highest roof surface.

[BG] HEIGHT, BUILDING. The vertical distance from grade plane to the average height of the highest roof surface.

Please note we’ve used the Wyoming Building Code for the examples above as it shares much in common with many other state and city codes. Please reference your jurisdiction’s codes for amendments specific to your project.