Vman

Does anyone know of any governing authority that talks specifically about Out-of-Scope changes. Our customers are wanting to spend additional money on a single award delivery order that was generated from our Multiple Award Task Order Contracts (MATOCs). NOTE: The delivery orders are issued on a rotational basis. The project is a new visitor center construction. However, the customers are wanting to add a shop building and pad, RV pads and a trail and boardwalk. These items were not in the original specs and drawings and are clearly out of scope. What authority can I show my customers to convince them that these items cannot be added?