Can I use a single-family home with similar square footage as a comparable?

You don't mention the square footage.  

Make sure you use the same square footage and type of home (don't compare a one story with no garage to a two story with garage for example).3.

Also, I'm interested in the average/typical costs involved with adding square footage to the house.  

You take two houses right next door to each other...both built 3 years ago for example, brand new (basically), same square footage...but one house is custom built - there is no house anywhere with the same floor plan, and the other house is a contractor special, with 3 other houses identical in the neighborhood.  

I created a spreadsheet that logged the purchase price and square footage of all the duplexes in the certain area code that I wanted.

You want to compare similar buildings structures, age, square footage, finishes, and rent rates to the one you are analyzing.  

Try to find a property built in the same year, same # of beds/baths, same features and updates, and within 20% up or down of the square footage.

In other words, comparing ranches to ranches, capes to capes, etc; keeping the square footage of the comparables to plus or minus 20% and BR and Baths similar.  

But if we are adding square footage, beds, bathrooms, etc., to a renovation, then the county is going to want a more in-depth analysis of the system and bring it up to code.Requirements vary by location...