There is a standard formula for most features (ie 2 vs 3 car garage or whatever). You can probably get that info online or from the appraiser him/her self.
If you're in an area without many recent good comps, the appraiser (if they're responsible) is going to value the house lower than what you might think it worth, because they are the ones making sure the bank has enough meat on the bone to make themselves whole. It *should* be a conservative estimate, most of the time. If there are lots of recent good comps and it came in super low, you can contest the appraisal.
-W