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.
A business plan is simply a plan of what your goals are for the business, and how you plan to go about accomplishing them. Some people refer to it as a vision for your business. I like to refer to it as a blue print or a road map to accomplishing your goals. Your plan should be spelled out in clear and defining terms and be kept simple. It should be a written document and used as a tool in managing the business.