Office Buildings are a scenario building, only obtainable via the scenario editor or in certain campaings/scenarios. They have an unused cost option and are all the same size as many other buildings such as the Library, Barracks and Refinery. It is interesting, that along with the Apartments, they all have 1275 health. 

Like other scenario buildings, office buildings cannot be constructed outside of the scenario editor and do not provide any benefits (execpt for landscaping purposes).