To enlarge the space, try lowering the floor, opening the ceiling or expanding out with box-bay windows. If you can put one in your attic or, by adding a second floor, consider stacking your bathrooms to cut costs. Properly placed, dormers and roof windows help offset the add-on appearance. Master suites have become a hot real estate feature. If you are converting a bedroom into a master suite, try to locate the closet as a buffer between bedroom and bath, and enhance the suite effect with recessed lighting, sconces and built-in adjustable reading lights.
Home offices are a growing trend, especially in predominately professional neighborhoods.
And while you're at it, consider making the office Internet-friendly. If the house originally had molding, it should be there. I believe in doing a house for yourself, not just for resale. Older houses where the owners have done a good job remodeling them sell quickly, even in neighborhoods where there's still a lot of rehab to be done. People can see past the decorating a lot of times, but if they see that a house hasn't been taken care of, they wonder what else is wrong that they can't see.