Creating residential square footage is an art and not a science. It
is an extremely complex issue and no one measurement standard
will work in every scenario. The commercial real estate industry
embraces seven measurement "standards." The residential side of the industry is still struggling with two. Although these two "standards" have been used by professionals for over a hundred years, the only name most people have ever heard of is the ANSI® standard. ANSI is an excellent reference guide and, when used in its entirety, provides consistent and recreateable measurements. But, for many real estate professionals, they have been taught to measure stairs (and the sloped spaces below) by a different method. Both of these methods provide acceptable measurements for use in appraisals and mortgage lending. The AMS® (American Measurement Standard) was created for the group of professionals who do NOT measure stairs by the procedures prescribed in the ANSI Guideline, but still need the protection of following a formal, definable, and defensible if necessary, Standard of Practice. The AMS allows practitioners to say with authority "this is how I measured the house." In the event of any square footage discrepancy, this document offers them the liability protection they need and should be afforded within their industry. Just because they do not adhere completely to the rules of ANSI does not mean they are not just as professional as those who do follow ANSI. It is simply a difference of philosophy that is common throughout the industry, much like the difference in philosophies that brought about the need for seven commercial measurement standards.
The main discrepancy in measurement methods has always centered around the measurement and calculation of stairs, the sloped areas beneath, and the measurement of upper level living areas. The measurement methods contained in the AMS are currently practiced by professional real estate agents, appraisers, and numerous industry leading home builders and architects (i.e., Centex Homes®, Donald Gardner Architects, Inc.® and by many nationally renowned designers featured on sites such as eplans.com®, etc.).
The AMS and its associated principles provide acceptable measurements for the FHA, VA, HUD, Fannie Mae and Freddie Mac. It is more a standard, not because a particular group or organization has endorsed or adopted its use, but because it is commonly practiced throughout the industry and accepted as a common method of calculating the square footage in single-family homes
http://www.housemeasures.com/ArticlePages/American-Measurement-Standard.html
The State of Michigan, requires the ANSI Standards.
http://www.costonappraisal.com/xsit...tent/uploadedFiles/How_to_measure_a_house.pdf