If you want to build a second-floor addition that is versatile and simply different, look into retractable roofs. Depending on size and materials, retractable roofs can run from a few thousand to tens of thousands of dollars. But retractable roofs also can save you money because you can use one room as an office, sun room, observatory and gym, based on your mood and changing requirements. In his article "Retractable Roofs for Your Home?," Marcus Pickett writes: "Polycarbonate (PC) sheets offer incredible advantages when applied to retractable roofs: Their light weight allows for easier mobile assembly, their UV resistance allows you to enjoy the sun without frying your skin in 10 minutes, their insulating value works for all climates, and they have comparable impact and fire-resistance to other forms of roofing."
For your second-floor addition, think about a hot tub under the stars. Suzanne Slesin describes in a "New York Times" article the benefits of a roof-top hot tub, explaining that couple Barbara Wyckoff and Peter Siris "pay little attention to the changing seasons, except to regulate the water temperature." Pressure-treated pine is a good material for deck construction. To avoid scorn from your neighbors, think about building the hot tub and surrounding structure in a location that will not create an eyesore for the neighborhood.
Think about building an upstairs bar and game room. Whether you like to kick back alone with a drink at the end of the day or spend your time in the company of friends, a second-story bar and game room with plenty of windows can be a cozy attachment or a distinct separate location, depending on your preferences. To get the most out of a second-floor bar and game room, consider the placement and size of windows and the relative positioning of the bar and other games (darts, billiards, shuffleboard) to those windows.