A notation is presented for formally describing the design of a user-interface. A specification of a system is given using the Object-Z formal specification language and then extended to describe presentation (user-interface design) using the User Action Notation (UAN). The semantics for the design are described using an Object-Z class library. The Object-Z notation is extended to enable designs to be described by adding UAN annotations to the specification. The extended Object-Z notation supports a design philosophy of abstract specification of functionality before development of a concrete user-interface design. By focusing attention on functionality initially, premature design effort may be avoided. Such formal specifications and designs are useful for documenting a user-interface design and enable analysis of the usability and safety of interactive systems.