You don't want to overimprove the house for the neighborhood and you may be better off buying a new home versus adding on to your existing home.
Also you have to consider the value of being able to enjoy having a 2nd bathroom until you eventually sell. It may save your marriage! lol..
All the best,
Alma Rose Kee, PA
Sold On Tampa Team
Future Home Realty
Value added is a difficult thing to project. But the addition of a second bath connecting to the master bedroom will definitely make your home more desirable and saleable. With this addition the home should sell for more. How much depends of too many other factors to be given serious consideration and would simple be "guess work."
It's probably best to view the addition from this perspective.
20% seems a very optimistic evaluation.
The end result depends entirely on the quality of the master bath addition AND competing homes for sale.
Will the added bath COMPEL the new owner to update the rest of the home (if needed)?
Will the master bath be porportional to the remainder of the home?
Is this home located in an age restricted community?
Your Realtor (r) will be able to compile the statistics regarding recent sales of 2/1 and 2/2 homes in your area to give you a real look at what the market is valuing most.
If you choose to proceed with the addition, make certain the quality of the work will endure the inspection of a discerning buyer.