I agree with Tyler, there is not enough specific information to answer that question.
If you have a 4 bedroom, 1,400 sq. ft. home, adding a bedroom to the existing space would detract a little of the value. However, if you have a 2,000 sq. ft. 2 bedroom home, adding a bedroom to the existing space will likely significantly increase the value of the home.
There are so many factors involved that need to be assessed. Contact a certified real estate appraiser or a very knowledgeable local real estate agent to consider all of the factors.
AnnieMac Home Mortgage
Certified Appraiser PA RL139711