If a document needs to be developed that requires input from other disciplines_Finance, HR, Property & Facilities, Marketing, Procurement/Supply Chain_ then most likely you are looking at a team building effort to get the job done. In any event, don't look at the task as only as a roadmap that leads to a profitable product or enterprise. Business plans are a great way to build team buy_in, force a thorough review of options, define objectives, establish benchmarks to judge performance, and help arrive at a plan_of_action. Ultimately, it can lead to a Project Management approach to implementing a plan and that can be as involved and detailed as is necessary.
The "Back of a Napkin" Plan: It is the source of entrepreneurial legend and lore, the million_dollar idea that was hurriedly scribbled on a bar napkin. Yet, for most potential business owners this option for business planning remains a fantasy. However, like any myth there is a tiny grain of truth inside. A quickie business outline can work as a launch plan under the right circumstances.