Before adding a bathroom in the $15,000 to $20,000 or more price range, consult a professional to make sure the upgrade will pay off with the appraised value. If a home has depreciated due to location or market values, you might break even adding on a brand new bath addition, but you might not.
Something else you need to consider is your home's bedrooms. More specifically, when is a bedroom not really a bedroom? Some homes have rooms without closets, and when it comes time to sell, owners find out that a room without a closet can't be listed as an additional bedroom. It can only be labeled as an office or a "bonus" room. Check local zoning ordinances, and if you do want to add a closet so your home can be valued as a three-bedroom instead of a two-bedroom, for example, research low-cost options and get plenty of estimates within your budget, as well as realistic forecasts of what the addition will do to the appraisal.