I've had run ins with appraisers that measure homes differently than is reported on tax rolls. I've never had any reason to doubt their methods until now. You can download the Kindle version of this book for under $10.00 and get a good solid education on measuring homes according to ANSI (American National Standards Institute).
In this age of some questionable appraisals, challenging them by mistakes in square footage can get some reviews. Reading and using this book as a source for that can come in very handy.