What is ANSI and Who Publishes the Standard?
ANSI stands for the American National Standards Institute, a private non-profit organization that oversees the creation of consensus standards in the U.S. Both Commercial and Residential standards have ANSI certified standards, but when people generically refer to an ANSI measurement standard, they are referring to the ANSI Z765 standard for measuring and calculating square footage that was originally developed by the National Association of Home Builders (NAHB) and is now maintained as an ANSI-approved document published by Home Innovation Research Labs. This standard outlines how to measure finished areas, how to treat stairs, openings, sloped ceilings, and more.

Colorado Requirements and Liability
In Colorado, like many states, requires licensed real estate professionals to measure residential properties in accordance with the ANSI Z765 standard when reporting square footage in MLS listings. While this rule primarily applies to real estate agents, accurate measurement is also important for appraisers, architects, and sellers. Incorrect reporting can lead to liability claims for misrepresentation—especially if a sale falls through or the buyer later disputes the stated size.

The 5% Rule
Colorado’s Real Estate Commission acknowledges that even with ANSI, slight variations can occur due to rounding, measuring conditions, or human error. As a guideline, a variance of up to 5% between reported and actual measured square footage is generally considered acceptable. Anything beyond that may require correction and could raise questions about the accuracy of the listing.

Due Diligence for Buyers and Sellers
Even with a standard, it’s wise for buyers and sellers to verify square footage independently. An independent measurement provides confidence, helps avoid disputes, and ensures that pricing reflects the actual size. This is especially important if there are discrepancies between tax assessor records, MLS listings, or online estimates.

Why Numbers Differ: Tax Records and Zillow
Many people are surprised to find that the square footage reported by the county assessor or shown on Zillow doesn’t match an ANSI measurement. Tax records often use older or incomplete building data, and Zillow’s “Zestimate” algorithms pull from multiple sources, which may or may not be based on a true field measurement.

Bottom Line
Whether you’re buying, selling, or appraising a home in Colorado, an ANSI-compliant measurement is the most reliable way to report square footage. It reduces risk, builds trust, and helps ensure a fair transaction for all parties.