Unfortunately, while it's pretty easy for many of us in the real estate business to give you a local "gross cost" per square foot for increasing the size of a home with and without plumbing, it's a lot harder to discuss atypical modifications such as changing the roof and the area under the roof in a home. Depending on the age of your home and the type of construction in the attic area, it can range from "sorta" expensive to a cost that may not be worth the effort. Everything hinges on the type of structure under the roof--are there rafters or trusses up there?
Your best means to determining costs for your home is to consult with a qualified architect, structural engineer and contractor in your area. Based on the type of home you own, and you particular desires for modifying the roof, these professionals will provide you with the most accurate costs and estimates to do the work you're proposing.
Grace Morioka, SRES
Area Pro Realty