It can't be followed because it's utter nonsense. It turns into convoluted, misleading report with adjustments exploding into triple digits.
It can't be followed. It is an atypical situation. They didn't want a 1700 sf GLA cape COD turned into a 200sf GLA because the main level living area was partially built into a hill on one corner, so only the 200 sf upper bedroom is the GLA and the main level is basement. That's just idiotic to think that's what they intended!
Here it is:
"Is there an exception process?
If the appraiser is unable to adhere to the ANSI standard, the appraiser
will provide the code “GXX001-” in the Additional Features field
on the appraisal form and must explain why compliance was not
possible. For example, berm homes with their entire square footage
below grade would be eligible for an exception. The appraiser must
provide justification for an exception and lenders are responsible
for confirming the appraiser provided an adequate explanation.
Fannie Mae will monitor for inappropriate use of exceptions (i.e., using
methods other than the ANSI standard for homes that have typical
above-grade square footage)."
You can't mess with typical above grade square footage. But you can use exceptions for atypical graded homes so that they are compared as they typically function.