The first step in developing a business plan is to conduct a market analysis of the area in which you are considering opening a day spa. By researching the market and putting a numerical figure on the actual size of your potential market (through assuming a "capture rate"), you will be able to properly size your facility on paper and generate financial projections that will determine its financial viability.
There are some surveys available that will detail the demographic and psychographic characteristics of the typical day spa-goer. For assessing information about your geographical market area, I like to use Claritas (800-866-6510, x650). This is the same firm that provides census-based marketing data used by America's largest and most successful companies for evaluating customers, sites, and markets.
Choose a qualified spa consultant to work with you on your business plan, at least for your market study section. When you go for financing, a business plan that includes an evaluation by a recognized industry consultant will carry more weight with lenders. Be sure to check the consultants previous projects and references.