There are many ways to measure square feet; gross, net, net net.
Condo,s most often include the propotianate share of the common space as well as the gross space of the actual apartment.
A private house is generaly gross square feet; outside wall to outside wall.
Co-Ops are measured by numbers of shares, rarely square feet.
Depending on what kind of property you have and the reason for needing to know the amount of square feet; you will get many answers to your question. But the best way is to measure it yourself.