Quick Answer
Measure the finished, heated, above-grade space at the exterior wall surfaces. Add up the area of every floor where the ceiling is at least 5 feet high (ANSI Z765). Exclude garages, unfinished basements, open porches, and detached structures. The result is Gross Living Area (GLA), the standard US residential square footage figure.
The ANSI Z765 Standard
ANSI Z765 is the American National Standards Institute's rules for measuring single-family residential floor area. It defines exactly which spaces count, which don't, and how to handle edge cases like vaulted ceilings, stairwells, and attached garages. Most US appraisers and an increasing number of MLS boards use it as the default.
The standard is voluntary — your tax assessor, your real-estate agent, and a builder may all measure differently. ANSI Z765 is the most rigorous of the common approaches, which is why appraisals and high-end listings cite it explicitly.
What Counts Toward GLA
- Finished, heated rooms on all stories at or above grade.
- Closets, hallways, and stairwells within the heated envelope.
- Stairwell openings count on the level the stairs originate.
- Bay windows if they protrude more than 18 inches.
- Vaulted-ceiling floor area at the lower level only — not double-counted on the upper floor.
- Slanted-ceiling spaces only where the ceiling is at least 5 feet high.
What Doesn't Count
- Attached and detached garages
- Unfinished basements, even if heated
- Open porches, balconies, and decks
- Screened porches that aren't fully heated
- Crawl spaces and attic storage
- Detached structures (sheds, guest cottages)
Below-grade finished space (e.g., a finished basement) is reported separately, not as part of GLA. Listings sometimes combine them into "Total Finished SF" but appraisers always break them out.
How to Measure Your House
- Measure the exterior of each floor at the wall surface using a 100-ft measuring wheel or laser distance meter.
- Sketch each floor plan as you go. Note every offset, bay, and recess.
- Calculate area floor by floor. Decompose into rectangles. Break L-shapes and bays into multiple shapes.
- Add the finished floors. Sum the above-grade levels into one total.
- Note exclusions. Record garage, basement, and porch areas separately on your sketch.
The US median new single-family home in NAHB data is about 2,200 sq ft, with the average closer to 2,400 sq ft. Use those as a sanity check.
Why GLA Numbers Disagree
The same house can have three different square footage numbers in three places:
- Tax record: often based on builder plans, may not include later additions. Frequently low.
- MLS listing: historically measured loosely by listing agents. Now tightening toward ANSI.
- Appraisal: measured by a licensed appraiser using ANSI Z765 or a similar standard. The authoritative figure.
Differences of 5–10% between these three sources are common. If a 2,400 sq ft listing comes back at 2,200 sq ft on appraisal, the lender will base the loan on 2,200.