What does a Class A Building mean? What are the differences between Class A, B and C buildings? When looking for new office space, a tenant will quickly realize most buildings are classified in one of those three categories. The factors that determine a building’s class vary in each market, so a Class A office building in a major urban city will be much different than a Class A office building in a small rural town of 30,000 people. There are no definitive formulas used to classify a building, but a general definition for each class is provided in the guide.