DWiley; "Of course MLS and public records contain errors. All databases contain errors. That does not stop them from being used as data sources."
Therein lies a HUGE part of the reason Values were artificially inflated as AVMS & BPOs achieved equal status as "non-appraisal" appraisals within the past 10 years (especially the past 8).
I suggest simply because they have been allowed to effectively replace 50%, or more, of traditional appraisals with interior inspection and hands-on measurement and ostensibly saved consumers and users $Billions became popular does not make their use in residential mortgage lending it reasonable, credible, nor right.
One example, of which there WILL be thousands more, is the concurrent thread by an Appraiser receiving a Demand for Payment from an attorney representing a consumer seeking damages for a Value Opinion based upon his 2055E utilizing MLS & assessment GLA of 3900sf v a prior appraisal citing approximately 500+/- lesser GLA based upon physical measurement. (true, all 3 MLS, assessment, and prior GLA estimates may all be incorrect). The only way to resolve that issue FROM THE OWNERS perspective is physical measurement by a competent Assessor, B&Z Inspector, or Appraiser. This case clearly demonstrates at least One Consumers' opinion of reliance upon "guestimates". We will see many more posts on this issue.
As noted above, MLS & assessment records routinely include, OR fail to note, finished basements and/or converted garages, enclosed porch conversions, &/or additions which may, or may not, be legally permissible uses.
I suggest that it is within the scope of appraisal license for an Appraiser to address these specific elements when either on-site, ""from the road", and during due diligence. Readily apparent variances between visual observation, MLS, assessment, prior appraisal GLA estimates should result in upgrading the assignment - or declining it.
In this thread, there are no readily available data sources per OP remarks. Upgrade or declination are the only credible options.
I suggest it is not within the scope of appraisal license for an Appraiser to be a Surveyor. It is Standard Appraisal Practice to rely upon either surveys, metes and bounds, and/or tax plat maps, and Topgraphy and Flood Maps etc. to estimate Lot Area, Utility, External Obsolescence and other factors. An Extraordinary Assumption applies to their estimated reliability.