Does adding rooms always add value?

[Joey Sargeant]

I have plans to add an additional bedroom in the basement of my house so I can bring in another room mate to cover my mortgage completely. 

I am curious with how beneficial adding rooms is to a property- when the plan for the property is to rent after I move out. I think I a bedroom is a always a good idea because I can get more in rent. However would adding a bathroom be beneficial? Bathrooms are more expensive to add and bring more risk to problems once it becomes a rental. It would be very convenient for me while I live in the house now to have a bathroom right beside my room, but I am trying to think long term and be smart with the money I put in. 

The same question goes for every area of the house. Do the benefits out way the cost when adding rooms or making big changes to a rental?