In the past I've written about the ANSI requirements regarding ceiling height and the prohibition of including an accessory building's square footage to the total square footage of the subject.  Another issue that comes up a lot, especially in the French Quarter, is how to define a single vs. multifamily home.  When you're looking at doubles or other multifamily units this is easy, but what if you're appraising a large French Quarter home with a slave quarter that has been carved into two small efficiency apartments?  Is it a single family with guest quarters or a multifamily home.  This is important because multifamily homes have traditionally sold for far less per square foot than single family homes.  However, the reason for the difference in square foot price doesn't apply here--multifamily homes have traditionally received less per square foot because they don't meet the requirement that home owners want--a large space that works for today's family's needs.  Here a large French Quarter home would meet that criteria and I'm leery of making the home into a multifamily just on the basis that it has an efficiency apartment.  There is no definitive answer--I look to see if the apartments are rented and if they have separate electric meters.  In other words I look at the owner's use of these spaces. I also look at tax records.  At the end of the day sometimes there is no definitive answer. The first question an appraiser is supposed to consider is "what is the highest & best use of the property."  If the answer is single family home with guest apartment I'm inclined to appraise as a single family.  Still though sometimes it's just a judgment call.