I'm planning a scenario in which my players, being a crafty lot, may try to destroy a building. Are there any rules for what it would take to accomplish that? Would a building have an AC?
A building would have an AC, although it would be fairly low. (A city wall was described as having AC 3 in the online d20 SRD.) More importantly, the walls of the building can be described as having a certain number of hit points and a level of hardness dependent on the materials used to construct it. When figuring the damage from an attack, reduce it by the material's hardness and apply the rest to the hit points of the structure's wall. When the wall's hit points are reduced to zero, it's destroyed.
There is a 3.0 edition book called the "Stronghold Builders Guide". It will give you everything you have ever wanted to know about any building. From thickness and hardness of various walls to different kinds of doors windows and floors to adding magic to the castle. I don't know how complicated your building is but this will help with the information you need.
Type the title into google and you will find useful links like the wizards web enhancement.