This post is full of excellent advice.  Homeowners should remember too that in homes today what might be considered a bedroom by one owner will be a home office for another.  There are other uses for a room in a house other than sleeping quarters.  But owners must be aware also that what is changed might need to be reversed to place the home on the market or be forced to accept a lower price.  If you watch any of the "home" shows on HGTV you'll see examples of really dumb stuff people do to their houses, some making the home almost unmarketable. It's your property to do as you please, yes, but to be a marketable one be careful of major changes.

To add value, add a bathroom.

Especially on any level which does not presently have one.