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How Big Is It?

When performing a Property Condition Assessment (PCA), we are often asked to confirm building dimensions. Within the scope of a PCA, this is generally a crude check to ensure that documents are substantially correct. The actual process of measuring a building is actually much more complex than it first appears.

Buildings may be measured for various purposes. These include regulatory compliance, use planning, tax assessment, calculating replacement costs for insurance purposes, and leasing, among others. Each tends to involve a different method, potentially resulting in quite significant deviations.

ASTM Has Five Definitions

The American Society for Testing and Materials (ASTM) has developed a Standard Classification for Building Floor Area Measurements for Facility Management (E 1836-01). This document provides the following five basic definitions:

  1. Building Exterior Gross Area. This is defined as the sum of the floor areas on all levels of a building that are totally enclosed within the building. The Gross Area includes all common areas and internal elements such as elevator shafts and columns. One subtle factor is that the measurement is based on floor area, not volume. For example, if a building projection extends from the floor to the ceiling, it adds to the building area. If it projects above the floor, such as from the wall as a bay window might, it is not counted as part of the area.
  2. Interior Gross Area. The Interior Gross Area is the Exterior Gross Area less the thickness of the walls.
  3. Facility Rentable Area. The Rentable Area is calculated by measuring the major vertical penetrations (e.g. elevators), interior parking, and void areas and subtracting them from the Interior Gross Area.
  4. Facility Usable Area. The Usable Area is that area that can be assigned to occupant groups. In addition to the previous exclusions, it does not include major vertical penetrations (columns), primary circulation (e.g., lobbies, hallways), and building service areas.
  5. Facility Assignable Area. Assignable Area is the same as Usable Area except that it includes interior walls, columns, and projections.

BOMA, ANSI, and SIOR Also Have Standards

One of the key uses of building area is to determine, on a consistent basis, the rentable square footage. The Building Owners and Managers Association (BOMA) has published two documents relating to office buildings and industrial buildings. The latest version of the Standard Method for Measuring Floor Area in Office Buildings (Z65.1) was approved by the American National Standards Institute (ANSI) in 1996. Standard Methods for Measuring Floor Area in Industrial Buildings was published as a joint effort between BOMA and the Society of Industrial and Office Realtors (SIOR) in 2004.

The two key measures contained in the office building standard (also applicable to retail establishments) are Floor Rentable Area and Usable Area. Rentable Area is defined as follows:

The result of subtracting from the Gross Measured Area of a floor the major vertical penetrations , where Gross Measured Area is defined as the total building area enclosed by the Dominant Portion, excluding parking areas and loading docks outside the building line. The Dominant Portion is further defined as the portion of the inside Finished Surface of the permanent outer building wall which is 50 percent or more of the vertical floor-to-ceiling dimension.

The Usable Area is the measured area of an office, storage area, or building common area.

Once these concepts are understood, BOMA describes a seven-step process for calculating the Rentable Area:

  1. Determine the Gross Building Area.
  2. Ascertain the Gross Measured Area.
  3. Establish the Floor Rentable Area.
  4. Measure the Usable Areas on each floor.
  5. Determine the Floor Common Area.
  6. Allocate the Floor Common Area to the Usable Area using the ratio between the Rentable Area and Usable Area.
  7. Allocate the Building Common Area to the Rentable Area using the ratio of the Building Rentable Area to the difference between the Building Rentable Area and the Basic Rentable Area.

The BOMA / SIOR standard for industrial buildings follows a slightly different methodology. Two general methods are offered. The Exterior Wall Methodology follows the exterior surface of each exterior wall and forms a perimeter of the building. Modern CAD tools are extremely useful for this purpose, especially when shapes are not perpendicular. The Drip Line Methodology follows the most exterior “drip line” around the roof system and forms a perimeter of the building.

With industrial buildings, one often finds mezzanine levels that may house office space or additional storage. The Mezzanine Area is generally added to the total Building Area, but it may be excluded by agreement of the parties. It is always important to understand the assumptions being used and clarify any exclusions.

Bringing the Standards Together

Acknowledging the differences between the BOMA and the ASTM standards, the ASTM committee (E 06) has formed a work group to reconcile the two standards. To date, however, no new document has been presented for balloting.

Net Rentable Square Feet 

The term Net Rentable Square Feet (NRSF) appears frequently and is of obvious concern to real estate investors. Although it is used frequently, the term appears to have no official definition. The default is the BOMA definition of Rentable Area. Despite the apparent simplicity of this calculation, it is unlikely that two people will come up with exactly the same Rentable Area. Therefore, BOMA states that any variance less than 2 percent is “deemed accurate.”

So How Big Is It Really?  

Depending on the purpose, calculating the area of any building can have a significant effect on rental returns and even operating costs for things such as insurance. We have seen errors propagated through documents and transactions, sometimes for years. Buyers, lessors and lessees are therefore advised to commission a building measurement study if dimensions appear questionable.

Volume 18, Number 2

August 2007

Copyright © 2007

The Engineering Advisor is intended to enhance your knowledge of technical issues relating to buildings.  For additional information on any subject, please feel free to call us.  Our commitment is to provide you with timely, accurate information.