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The ANSI Z765 standard for calculating square footage.


You would think that measuring a home is a straightforward endeavor. That's what the National Association of Home Builders (NAHB) thought almost two decades ago when it conducted an informal survey of over a dozen builders. The survey results, however, produced over a half-dozen different answers! What seemed straightforward actually was not.

NAHB realized there was a need for a common method of measuring and calculating the square footage of homes. In 1994, it commissioned the NAHB Research Center to develop and recommend a systematic measuring method. The research center gathered input from over twenty-six real estate organizations. The final result of this effort was the American National Standards Institute (ANSI) standard Square Footage--Method for Calculating: Z765-1996. (1) The current version of the standard is ANSI Z765-2013. (2)

The ANSI Z765 standard is a voluntary guideline for describing, measuring, calculating, and reporting area for single-family homes. This standard is not meant for multifamily homes or commercial properties. (3) The ANSI Z765 standard is applicable for measurement of detached single-family residential homes with open space on all sides. It is also applicable to attached single-family residences such as duplexes, row houses, and townhouses.


Everyone knows how to measure, but what area is included in the measurement? There are many different ways to calculate the square footage of homes. Assessors, appraisers, and realtors may use different rules for measuring and reporting single-family residential area.

Assessors perform mass appraisals and usually have no access to the interior of the individual homes. They may not have access to the plans and specs of homes. Typically, assessors measure the exterior perimeter and determine the area by summing the above-grade levels. This works if the home's levels are equal. But, what happens to the accuracy of the calculated area if there are two-story foyers, open atriums, or a sloped roof on one or more levels, as in the case in a bungalow or Cape Cod-style home? Since the assessor does not have access to the interior of the home, many assumptions are made. These assumptions are opportunities for inaccuracies of above-grade and below-grade measurement.

Realtors use a different methodology when determining area. Instead of measuring the exterior perimeter of each level, they measure and report individual interior room area. Only major rooms--such as baths, bedrooms, dining rooms, family rooms, kitchens, living rooms, and recreation rooms--are included in the reporting. Ideally, summing all the individual interior areas, including hallways, pantries, and other subordinate rooms, closely approximate the total exterior area for each level. Realtors also frequently use the tax-assessed area for the overall area of both above-grade and below-grade areas. As previously mentioned, the tax-assessed area is often an estimate based on the exterior of the homes, without verification of the interior. Consequently, this approach results in incomplete reporting of the total area.

The appraiser's approach for determining area is a blend of both the assessor's and realtor's methods. Appraisers usually access the exterior and the interior of a house. The appraiser's approach is similar to the assessor's approach since the external perimeter is measured. It is similar to the realtor's approach since the interior is inspected to account for invalid spaces in the area calculations. Interior inspections may require the appraiser to measure interior dimensions as well. The appraiser's approach is superior to both other approaches, but appraisers often report different square footage for the same house, because different individual appraisers use different heuristics when measuring.

What's missing is a common methodology. It's a common methodology that provides consistency and repeatability. A methodology provides guidance on how to consistently measure specific attributes of a home, such as a bump-out or an enclosed three-season porch. The lack of a consistent methodology produces varying results when reporting a home's area, which causes confusion with clients, questions, and a lack of credibility.

Measuring disagreements are main causes of appraiser litigation. Standardization of measuring methods clarify how something is measured, so there's less chance of appraiser litigation. Using a common standard across an industry reduces confusion and increases credibility. There are a number of measurement standards in use, such as those of the Employee Relocation Council (ERC), ANSI Z765-2013, and the American Measurement Standard (AMS) C42129-2009 used by the US Department of Housing and Urban Development. However, ANSI Z765 is the only one of these standards that is supported by two dozen industry organizations, peer reviewed, and recommended for use by over a half-dozen states. (4)

Common Causes of Inaccurate Measurements

Measurement errors impact the area that is reported by appraisers. There are a number of common causes of inaccurate measurements, including errors during the measuring process; missing above-grade area; counting non-above-grade area as above-grade area; complex floor plans; and the inability to measure the exterior of the home.

Errors in the measuring process can be the result of equipment failure. Equipment failure is caused by things related to equipment performance. A damaged or warped measuring wheel or a rock in the wheel can cause significant measuring error over long distances. Another example of equipment failure involves a laser measuring tape. When battery power levels get low, the laser measurer may produce readings that have significant error. These examples illustrate that proper maintenance of equipment is an essential part of preventing measurement error.

Measuring error also can be caused by a lack of squaring. Squaring is the process of ensuring that measurements on one side of the building equal the measurements on the opposite side of the building at the time of inspection. A problem occurs when the measurement error is discovered later. Squaring measurements is straightforward if the property is a square or a rectangular house. Measurements on the left side equal those on the right side and the same for top and bottom. Squaring is not so simple, however, if the subject is a complex property with many curves, for example, a Victorian house with a series of angles around the perimeter. Squaring complex dwellings is difficult in the field by hand, but it is doable using some of the newer technology, such as tablets or smart phone applications with graphing capabilities.

Measurement error also can be caused by missing above-grade area. This error occurs when an appraiser misses valid above-grade area. Some examples of missed areas are four-season porches, bonus rooms, and the exclusion of stairs and hallways that extend to unfinished spaces, and mother-in-law and auxiliary units attached to the home via finished area.

The opposite can also occur when non-above-grade area is counted as above-grade area. Examples of this kind of measurement error are the inclusion of three-season porches, decks, patios, unattached auxiliary buildings (such as a mother-in-law unit above a detached garage), attic spaces, garages, below-grade area, open foyers, atrium area, and unfinished stairs.

Complex floor plans produce the most confusion in measurement. The confusion arises from the inconsistent measuring by appraisers of special cases of these kinds of homes. For example, how to compute above- and below-grade area when 32% of the first-floor level is below grade. Appraiser A may decide to treat the first level as all above-grade, while Appraiser B treats the first level as all below grade, and Appraiser C treats 68% as above grade and 52% as below grade. Which appraiser is correct? The short answer is they all are unless there is some kind of agreed-on standard and uniform consensus on how to measure these special cases.

Finally, measurement error can be related to the inability to measure the exterior of a home. This occurs in situations where it is impractical to measure the exterior, such as attached townhouses and upper levels of detached homes. Two approaches are used in this situation: either the interior perimeter measurements are taken or the interior perimeter is measured and the wall thickness is added to the measurements. Both measuring approaches are not consistent with measuring the outside perimeter. Adding wall thickness to the inside perimeter is an attempt to approximate the outside perimeter measurement method, but estimating the wall's thickness produces uncertainty with this approach. Either measuring approach in this scenario introduces the possibility of errors.

With the exception of equipment-related issues, all the causes of measurement error are the result of appraiser interpretation. Specifically, the measurement errors occur when the appraiser has to use his or her individual judgment for determining how to handle special situations. This is where using a measurement standard is helpful. A standard provides guidance on procedures and methods for measurement in various circumstances and for reporting the results. The ANSI Z765 method for calculating square footage is such a methodology.

ANSI Z765 Overview

There are three sections to the ANSI Z765 standard. The three sections address the definitions used in the standard, the methodology for measurement and calculation of square footage, and the reporting of a statement of square footage. An important point in using the standard is that adherence to all three sections is necessary in order to claim ANSI Z765 compliance; noncompliance with any portion of the standard renders the entire calculation and report noncompliant. ANSI Z765 also has an annex section, which offers commentary on use of the standard; however, the annex is not considered part of the standard itself.


ANZI Z765 defines eight key terms: (1) attached single-family house, (2) detached single-family house, (3) finished area, (4) unfinished area, (5) garage, (6) grade, (7) level, and (8) square footage. The definition of each term is as follows.

* An attached single-family house is a dwelling that "has its own roof and foundation," and "is separated from other homes by dividing walls" Under the definition, the dividing walls must extend from the foundation to the roof. Also, the home cannot share any of utilities with an adjoining house. Attached single-family houses include townhouses, row houses, and duplexes.

* A detached single-family house is "a house that has open space on all its sides" This type of home does not share common walls with any other house, is not connected in any way to any other units, and does not share utilities.

* Finished area is "an enclosed area in a house that is suitable for year-round use" The finished area has walls, floors, and ceilings with finishes similar to the rest of the house. (5)

* Unfinished areas are sections of a house that are neither finished nor suitable for year-around use. Patios, decks, porches, balconies, garages, and carports are examples of spaces that have varying degrees of finish but are not suitable for year-round use, and thus are considered unfinished areas. A laundry room that has exposed ceilings and concrete floors is considered unfinished, while a laundry room with finished walls, ceiling, and stamped concrete flooring is considered finished. The standard's annex section provides further discussion on what constitutes a finished versus an unfinished space.

* A garage is an alternate utility structure that is "intended for the storage of automobiles and other vehicles."

* The grade of a home is "the ground level at the perimeter of the exterior Finished surface of a house."

* A level is "the areas of the house that are vertically within two feet of the same horizontal plane" If a level differs by more than two feet in the horizontal plane, then the area must be considered different levels.

* The term square footage is used in the standard to denote area. Square footage is defined as "an area of a house that is measured and calculated in accordance with the standard" Units of area are square feet in the English measurement system and square meters in the metric or Standard International (SI) system. The standard states that when employing metric or SI measurement units, the term floor area is used in place of square footage. (In the discussion in this article, the terms square feet and area are used throughout as a unit-neutral terms.)

Commonly used terms like gross living area (GLA) and below-grade square footage (BSF) are not used in ANSI Z765; these terms are defined in HUD 4150.2, Valuation Analysis for Single Family One-to Four-Unit Dwellings. The ANSI Z765 standard committee chose not to use GLA or BSF because it might cause confusion in the marketplace. Instead, the committee chose to adopt the generic terms of finished and unfinished square feet as well as specifying it as above grade and below grade.

Area Measurement and Calculation

To adhere to the standard, the proper area units are necessary for measurement and calculation. When using the English measurement system, units are measured to the nearest inch or tenth of a foot, with final square footage reported to the nearest whole square foot. When metric or SI measurement units are used, the house is measured to the nearest 0.01 meter, and the final floor area is reported to the nearest 0.1 square meter.

Besides specifying units, the ANSI Z765 standard provides important guidance for measuring specific elements of a home. The standard's commentary discusses how to calculate square footage of finished areas adjacent to unfinished areas, openings to floors below, above- or below-grade finished areas, ceiling height, unfinished areas, and protrusions. The following explains some of these elements in detail.

For attached homes, the finished area on each level is the sum of the finished areas measured to the exterior finished surface of the outside wall, or from the center lines between the attached houses. Measurement of the finished area in detached homes is the sum of each finished area on that level to the exterior finished surface of the outside walls. The measurement of above- and below-grade finished area is a variant of the detached method just discussed. Specifically, levels that are entirely above grade are considered above-grade finished area, while below-grade finished areas of a home are the sum of the finished areas on the level that are wholly or partially below grade.

The standard has specific guidelines for the measurement of openings and stairwells. Openings to the level below are not included in the area calculations for that level. Examples of open areas are two-story foyers and atriums. Stair area is counted as square footage for the level from which it descends. Many new construction homes provide a level opening that is equal to the exact width of the stairs. Thus, the area of the stairs included in the finished area is typically equal to the area of the opening of the level.

Moreover, stairs descending to an unfinished basement are included in the finished area of the first level regardless of the degree of finish of the stairs or the degree of finish surrounding the stairs. For the level where the stairs terminate, the areas under the stairs are also included in the finished area regardless of the distance between the stairs and the level below. This is a quirk in the standard, since the implication is that the area under the unfinished stairs descending to an unfinished basement is considered finished.

The standard's commentary also provides guidance for how to measure finished areas adjacent to unfinished areas and finished areas connected to the house. A finished area adjacent to an unfinished area on the same level is calculated by measuring to the exterior edge or the unfinished surface of any interior partition between the areas. Finished areas that are connected to the main body the house--such as hallways, breezeways, or stairways--are included in the finished area of that level. Alternatively, a finished area that is not connected to the house (for example, a four-season covered patio) may not be included in the finished area of any level of the home.

Additionally, the standard provides distinct guidance on what is an unfinished area. A garage is an unfinished area and cannot be included in the calculation of finished area. Chimneys, bay windows, and other finished areas that protrude beyond the exterior are not considered finished area either. There are some exceptions to this rule; for example, a protrusion that is finished and extends from the floor to a minimum ceiling height of 7 feet may be included in the finished area.

Finally, the standard's commentary provides precise guidance on ceiling height requirements. For a space to be included in finished area calculations, the finished area must have a ceiling height of at least 7 feet (2.13 meters) except under beams, ducts, and other obstructions where the height maybe 6 feet 4 inches (1.93 meters). As previously mentioned, space beneath the stairs is considered finished and has no specified height requirements. If a room's ceiling is sloped, at least one half of the finished area in that room must have a vertical ceiling height of at least 7 feet. No portion of the finished area that has a height of less than 5 feet (1.52 meters) may be included in the finished area measurements. Figure 1 illustrates the ceiling height guidelines.

Reporting of Area

Once the various areas of the home are measured and calculated they must be reported.

ANSI Z765 specifies how units are reported. All reported areas are rounded to their respective unit of measure. So, in the English measurement system results are reported to the nearest whole square foot. Alternatively, units are reported to the nearest tenth of the square meter or square decimeter when using the SI units of measurement.

The reporting section of the standard states that all above- and below-grade finished areas must be clearly and unambiguously reported in the Statement of Finished Square Footage. Specifically, the standard states that a home's finished area cannot be reported without a "clear and separate distinction of both above-grade areas and below-grade areas."

The standard contains guidelines on four issues related to reporting of area: (1) areas not considered finished, (2) exterior-only inspections, (3) plans-based inspections, and (4) reports based on estimated measurements.

Areas that are not connected to the home, unfinished areas, and other areas that do not fit the requirements of finished area are not considered finished but may be listed separately. Any calculations and statements of unfinished area must distinguish between above-grade areas and below-grade areas.

Ordinarily, an interior and exterior inspection is the norm when using the standard. If an interior inspection is not possible to confirm finish areas, unfinished areas, or openings in the floor, then the standard requires a declaration statement indicating there was an exterior-only inspection. The declaration must be similar to the following:

   Finished square footage calculations for this house
   were made based on measured dimensions only and may
   include unfinished areas, openings in floors not associated
   with stairs, and openings in floors exceeding the area
   of associated stairs.

If the reported area is based on plans and specifications for a proposed house, the report must include a declaration clearly and explicitly stating that the calculation is based on plans. According to the standard the declaration should be similar to the following:

   Finished square footage calculations for this house were
   made based on plan dimensions only and may vary from the
   finished square footage of the house as built.

There are some situations where direct measurement of the structure is not possible for various reasons such as access, nature of the terrain or structure, or other obstacles that preclude direct physical measurement of the exterior. The standard recognizes that dimensions developed through means other than direct measurement or through plans and specifications are susceptible to inaccuracy, as is the resulting calculated square footage. Consequently, calculations developed under such circumstances must be explicitly identified and reported with a declaration similar to the following:

   Finished square footage calculations for this house were made based
   on estimated dimensions only and may include unfinished areas,
   openings in floors not associated with stairs, or openings in
   floors exceeding the area of associated stairs.

Failure to provide the required declarations, when applicable, voids any claim that the report adheres to the standard.

Calculation Examples

Now that we have discussed the standard in detail, let's illustrate how it works in practice with three examples. These examples use the ANSI Z765 standard to measure the same home; however, in each example one or more of the parameters are changed.

Let's begin with a description of the case study home's features and dimensions (Figure 2).

The subject home is a two-story colonial detached single-family house, with an unfinished, walk-up third level and a basement. Its foundation is 28.2 feet wide and 42.5 feet long. The first and second levels are equal. The third level, however, has gabled windows with a sloped-roof pitch 4 feet in height. The rest of the third level has a sloped-roof pitch along the 42.5-foot length with a height of 5 feet that rises to an apex of 12 feet in height. The width of the main area of the third level is 20 feet. The first level has a two-story foyer that is 10 feet by 10 feet, an unfinished utility room that is 8 feet by 12 feet, and an enclosed three-season porch attached to the rear of the home that is 28.2 feet by 10 feet.

The basement has an unfinished storage room that is 20 feet by 22 feet. The rest of the basement is finished with 7 feet 5 inches floor-to-ceiling height throughout. Additionally, the home has a detached two-car garage that is 20 feet by 20 feet.

Using the dimensions and the description provided, for Example 1 the statement of finished area for this house would be as follows:

Example 1 Declaration

A 28.2' x 42.5' two-story detached single-family home has 2201 above-grade finished square feet (SF), plus 96 above-grade unfinished SF in a utility room. The basement level has 759 below-grade finished SF, plus 440 below-grade unfinished SE The home also has a 280 SF above-grade enclosed porch as well as a 400 SF detached garage.

Here, the first level less the utility room is calculated as above-grade area. Also, all of the second level less the two-story foyer is added to the first floor. The below-grade area is the foundation area less the area of the storage room.

Now, suppose in Example 2, the home is the same as in Example 1 except 20% of the home's first level is bisected by a hill, and the porch is completely above-grade. The statement of finished area for this home is described in the following:

Example 2 Declaration

A 28.2' x 42.5' two-story detached single-family home has 1099 above-grade finished square feet (SF). The first and basements level have 1958 below-grade finished SF, plus 536 below-grade unfinished SE The home also has a 280 SF above-grade enclosed porch as well as a 400 SF detached garage.

In this example, the first level is considered below-grade since the ANSI 765 standard states that if any portion of the level is below-grade the whole level is considered below-grade.

In Example 3 there is a different twist. In this example, the hill from Example 2 is gone, but now the third level is finished, and basement ceiling height in the finished area is 6 feet 3 inches in over 60% of the finished area. The statement of finished area is described in the following:

Example 3 Declaration

A 28.2' x 42.5' two-story detached single-family home has 3051 above-grade finished square feet (SF), plus 96 above-grade unfinished SF in a utility room. The basement level has 304 below-grade finished SF, plus 895 below-grade unfinished SE The home also has a 280 SF above-grade enclosed porch as well as a 400 SF detached garage.

In Example 3, the first and second level area is the same as Example 1, plus the third level for the above-grade area. The finished below-grade area is the product of the 40% of the below-grade finished area of Example 1.

As you can see, minor changes in the parameters result in radically different results using the standard. These examples are good representation of the standard since they use key components on which the standard provides guidance. Another key point is that all square footage included in the report must be calculated using the standard's methodology, otherwise it voids an claim of adherence to the standard.


The ANSI Z765 standard provides guidance for many measurement conundrums. Moreover, it provides consistency and repeatability. The use of this standard can reduce confusion and inconsistent measurements among practitioners. Using the standard in appraisal reports decreases liability and increases credibility of measurements within the appraisal profession.

Additional Reading

Fannie Mae. Selling Guide: Fannie Mae Single Family, Section 405.06.

Home Innovation Research Labs. Square Footage-Method for Calculating: ANSI Z765-2013.

Thomas, David Hampton. The American Measurement Standard: Calculating Residential Square Footage.

Institute of Housing Technologies, 2009.

Thomas, David Hampton. Size Matters! Solving the Square Footage Puzzle--Appraiser's Edition. 2009. US Department of Housing and Urban Development. Valuation Analysis for Single Family One- to FourUnit Dwellings, Handbook 4150.2, Sect. 3-3A.

Wright, Larry T. Review of Size Matters! Measuring and Calculating Residential Square Footage. The Appraisal Journal (Summer 2008): 285-286.

(1.) The American National Standards Institute does not develop any standards itself. Rather, it accredits standard developing organizations (SDOs). An ANSI standard is a voluntary consensus created by the experts participating in the SDO and approved by the ANSI Board of Standards Review as meeting certain criteria in the development process. ANSI, What Is ANSI? An Overview (Washington, DC: American National Standards Institute).

(2.) In 2013, the NAHB Research Center was renamed Home Innovation Research Labs; ANSI Z765-2013 can be purchased on its website at

(3.) The Building Owners and Managers Association (BOMA) has had measurement standards for office buildings for almost 100 years; the current version is Office Building: Standard Methods of Measurement (ANSI/BOMA Z65.1-2010). There are also ANSI/BOMA measurement standards for other building types, see

(4.) Although ANSI Z765 is generally a voluntary guide, its use is may be required by local, state, and federal government entities, such as Fairfax County (Virginia), the states of Alabama and Kentucky, and the US Air Force.

(5.) The commentary section of ANSI Z765 notes that the term habitable space is often used in building codes to describe space that has as one of its requirements a specified amount of natural or mechanical light and ventilation. The ANSI term finished area, however, does not imply that finished space conforms to any requirement for light and ventilation, and there has been much discussion on this. The 2012 International Residential Code (IRC) defines habitable space as "a space in a building for living, sleeping, eating or cooking." Under the IRC, habitable space excludes bathrooms, toilet rooms, closets, halls, storage or utility space, and similar areas. International Code Council, International Residential Code (International Code Council, 2011), 17.

Byron Miller, SRA, RAP., MSSE, is a certified residential appraiser in Minnesota and Wisconsin. His firm BM Appraisals specializes in valuation services for consulting, estate, litigation support, mortgage, and relocation. Miller has ten-years' appraisal experience, and previously was a computer engineer. He has taught prelicensing appraisal courses, and he is a member of the Board of Directors of the North Star Chapter of the Appraisal Institute. Besides his designations from the Appraisal Institute and the National Association of Realtors, he has an undergraduate degree with a focus on computer science and a master's degree in software engineering. Contact:
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Author:Miller, Byron
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Geographic Code:1USA
Date:Sep 22, 2013
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