There are several variables that affect value. However, two of major importance are 1) the location of your house. You don't want to price yourself out of the neighborhood. 2) Would such a huge room over shadow the existing Master BR. I personally don't think you have to physically construct the closet. The house can be advertised as 3 BRs with possible fourth. Your agent can easily point out out to 4 BR buyer how easy it would be to convert to 4th BR. Depending on what I mentioned in #1, you may want to think about converting the bonus room to a MBR and add a bath, closet, etc to make it a master suite. This type of addition has great appeal. Of course, I make that suggestion not knowing a whole lot about your other rooms and the potential your house has of absorbing this added expense. Just something to think about.