Many times brokers and sellers need to estimate the unit square footage and all they have is the total building square footage. So the question is how much to deduct for the exterior walls. The general rule of thumb is 4% of the total square footage.
Let’s say we have a structure that is 100′ x 100′ for a building totaling 10,000 square feet. A 12″ thick wall x 100 SF x 4 (assume there are 4 walls) equals 400 square feet of exterior wall which translates to 4% of the total square footage.