How do lenders differentiate between multi family and single fam

[Katie Stroman]

Looking to buy my first property and will have my parents as non-occupying co-signers (I will be occupying the home though). Looking at the Home Ready program through Fannie Mae. I understand when having non-occupying co-signer on a 2 to 4 unit property you need 15% down. My question though is how do they determine what is  considered multi family and what is not? I'm looking at a property that has R2 zoning it is listed  as a single-family with an accessory unit in the public records. That accessory unit is a four car detached garage with a two bedroom apartment on top and does have a permit from the township to be occupied. It also has separate utilities but shares the septic system. Could  this come back as a two unit property in the appraisal therefore causing me to put down 15% instead of 3% ? Every lender I've talked to has said it's a gray area and that we won't know until the appraisal comes in. Thanks in advance for any advice/insights/personal experiences.