Will your current neighborhood support the value of your home after you add on? You never want to over improve if the neighborhood can't support the cost. You are smart to do lots of research. Good Luck

A remodeling project of that size will almost certainly be an expensive proposition. By the time you factor in all the costs to upgrade or add systems (heating, cooling, plumbing and electrical are big on the list) as well as the economy of scale required, it may be more cost effective to do a tear down and rebuild. Not necessarily cheaper, but more cost effective. New things, like energy efficient heating and cooling, solar, water filtration and upgraded electrical systems, smart house wiring and super insulating can be incorporated in the new work as a global concept as opposed to cobbling together the pieces. A consultation with a realtor and an architect, with input from a general contractor is a good way to start.