What is ANSI?
If you are looking for information about ANSI/BOMA the commercial measurement standards: go here.
ANSI (the American National Standards Institute) is a private, non-profit organization that accredits the development of voluntary consensus standards in the United States. It does not itself publish most standards, but rather approves the processes used by standards‐developing organizations and grants the “American National Standard” designation.
BOMA’s measurement standards are ANSI certified, but when people ask about the ANSI measurement method, they are often referring to a widely-used measurement document in residential real estate: ANSI Z765-2021 (“Square Footage – Method for Calculating”), published by Home Innovation Research Labs (formerly affiliated with NAHB). This standard establishes how to measure and report the Gross Living Area (GLA) of detached and attached single-family homes, including rules for ceiling height, sloped ceilings, stairs, basements, and finished versus unfinished area.
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Colorado Requirements and Liability
In Colorado, real estate licensees are not required by statute to measure every listing themselves, but the Colorado regulation advises that if exact square footage is a concern, an independent measurement is recommended. MLS providers (such as REcolorado) may use their own rules, which do not necessarily require ANSI Z765 conformity. Inaccurate square-footage reporting can expose sellers, agents and brokers to liability for misrepresentation.
Variance and Due Diligence
While there is no formally mandated “5% rule,” many practitioners use an industry guideline of up to a 5% variance between the reported area and an independent measurement as generally acceptable. A larger discrepancy may raise questions and trigger disclosure or negotiation. BOMA Standards however outline 2% as an acceptable variance. I have even heard it argued that with modern measurement practices, we could significantly reduce what is an acceptable tolerance between measurements.
Why Numbers Differ: Tax Records and Zestimate
County assessor records and automated tools like Zillow’s Zestimate are often based on older data, tax-qualified definitions, or algorithmic estimates, and may not follow ANSI-compliant measurement methods. As a result, their figures can differ from a measurement done according to ANSI Z765.
Bottom Line
For buyers, sellers and appraisers in Colorado — when measuring or representing square footage of a single-family home — following ANSI Z765 (or retaining a professional who does) is considered the most reliable and defensible approach. It supports clarity, accuracy and confidence in the transaction