This paper describes a design space analysis approach towards a "complete" planning solution. A complete solution is defined as one containing the resultant plan, the context in which it applies, and the argument structure that justifies it. The focus in this paper is on defining and communicating the argument structure component. A perspective of a plan as a specialised type of design and planning as a specialised form of design activity is used. In doing so, research is drawn upon from the design rationale community for generating an explanation of a designed artifact. In particular, the method of generating a design space which represents the location of the plan within the space of possible plan elaborations is adopted. An initial implementation, Nonlin+DR, is described and its potential benefits to stand-alone and mixed-initiative planning is discussed.