Although adding that bathroom might help to improve the quality of life for someone living there, I doubt that it would add an additional $20K to your sales price if you were to sell a year later. Contrary to all of the garbage that's been put out there via some design shows, the reality falls more in line with what Alan initially stated: "It's going to depend largely on the properties available in your area, as to whether this makes sense to do, or not."
If most of the other houses in your neighborhood have at least 2 baths, then you might want to consider adding one to be competitive. Yet, adding another bath isn't the only way to be competitive. You could add more value to your future listing by offering to sell for a slightly lower price than your competition, offering more flexible terms, including something like a gift card to a restaurant or store for the new future owner, etc. Plus, you'd be more likely to receive a higher return for actual dollars spent.
It depends on your market place and how much square footage your home has. $20,000 is the very top end of the price range for this project and that would be close to my expected return on investment. You might be better served to do a half bath upstairs for a lot less money and still would yield a ROI on you investment. Best
I doubt you'll recover the full cost of paying for a second bath, but if you do it properly, it will make your home much easier to sell. I can tell you that I would never purchase a home with only a single bath again, unless it was a one bedroom condo.
A three bedroom house is designed for a small family, and a small family is going to want (if they can afford it) at least a bath and a half.