Factors determining number of units

[Ray Cummins]

I'm a newbie trying to get into real estate investing and I have a question regarding who/what determines the number of units a property has, as it relates to the type of loan you can get.  

I know I've heard of situations happening before where people will buy a quadplex and upon viewing, it actually turns out to be a 5-unit property (which could've been the difference between a residential vs commercial loan). From my own experience, I've looked at a property that was classified as duplex on the county appraisal database, listed as a triplex on the MLS, and when viewing in real life it was a quadplex (luckily still qualifying as residential). Another situation I've found is a property listed as a triplex on the MLS, the county appraisal listed it as a Three Family house, but further down the appraisal I'll see the property having both a Triplex and a Duplex on the property.

Where some people are able to find some hidden value in the property, others are walking into what they thought would be covered by a residential loan and finding something that will require a commercial loan.  Appreciate any help or clarification you guys can provide.