Square Footage Discrepancies When Listing
The goal with regard to advertising square footage is to advertise the accurate value. Generally, the county appraisal district, a builder, and an appraisal report are considered accurate. It's very normal to see discrepancies between these values up to roughly 10%. Beyond that, and you might want to hit pause and question "What's the real square footage? Should we send an appraiser out to verify?"
Cubicasa App: This is a free floor plan app available through ABOR and others. It does provide square footage values that are generally accurate. Most agents and buyers won't consider this a trusted source, but it's a great place to start and then get a verification by an appraiser if you need an "official" verification. https://www.cubi.casa/pricing/
Different Values from Trusted Sources
It's pretty normal to see SqFt discrepancies up to ~10% (the bigger the discrepancy, the more you may want to investigate, though.) When you have more than one value from a trusted source, you generally want to advertise the higher amount, unless you have reason to believe that higher value is not accurate.
If you use a value other than the county appraised value, you always want to upload verification as an MLS doc - the square footage page of the appraisal or the builder's verification.
FAQs
What is considered a reliable source for square footage? The county appraised value, a third party appraisal, or a builder verification (like plans) are all considered reliable and buyers / buyer agents will trust them.
What about owner reported square footages? "Per owner" is generally considered inaccurate when listed as the source on the MLS. You can have an appraiser verify square footage for a few hundred dollars, which is always worth doing.
What if there's an addition that wasn't reported to the county? The first question you should ask is "Was this permitted?" which will open up another set of problems to solve if that's the case. It's not uncommon for the county to be unaware of a permitted addition. If that's they case, then you need to have a set of plans or an appraisal to verify the square footage.
Why is the county appraisal district wrong and how often does that happen? There are lots of reasons why the county can be wrong about square footage. Humans make mistakes and humans enter the data. Sometimes things change (plans, additions, etc) and the county doesn't update it. You'll see incorrect values at the county on 2-3% of listings (that's a guesstimate).