Home Buliding and Interior Design

As an appraiser of real estate, I come across attached dwellings that are classified as single family residences, and detached dwellings that are classified as condominiums. What is the criteria for determining whether a new project or tract will be classified as either condo’s or Single family residences. I’ve heard everything from the number of units per acre, to quality of construction, to how sewer lines are hooked up. I’m familiar with the legal definition for both types of ownerhsip but can’t figure out why for example you might come across a tract of detached dwellings (homes) but there legally classified as condominiums. Any information on this would be greatly appreciated.