When you're writing a business plan in a situation like these, you need to address a few issues the intentional entrepreneur has already pondered. The first is do you really want this idea to become a full_blown business? Certainly it's flattering when you realize there's a market value for something you were doing anyway, but that doesn't always mean you should launch a business. A lot of accidental businesses form around fads or seasonal items, and may not be robust enough to function as year_round, money_making, enterprises.
You would first do a P&L for the present year for your existing business and the first year and as many years after as you would like to have your plan cover. Your existing business financials will be the foundation for building yourself a business plan for as many years out as you want. This data will tell you a number of things but first if you want to build your plan around what you want in life, you would need to decide some things about your life: 1. You would need to decide how much income you would like to have for yourself for each of the years you plan for. 2. You would need to determine what kind of profit margin you would want from your business for each of the years. 3. And by combining these 2 things into a P&L format you can develop a financial business plan that can extend as for into the future as you would like. 4. The first thing it will show you is how much sales you would need each year to give you the income and profit you would like. Once you see the sales needed, if you know your business well enough, you should be able to estimate those additional expenses needed to overcome capacity constraints that will occur as your business grows.