You ask a tough question because you haven't provided me with any specifics. When you say 2nd, are you talking about a 2nd floor, a 2nd bedroom or a 2nd bathroom, or some combination of those? Because of the nature of your question, I am assuming that you do not yet have architectural drawings for the project you envision.
As a rule of thumb, remodels and additions are more expensive on a cost per square foot basis because of the cost of modifying the existing structure to accomodate the addition. For budgetary purposes you can probably use a number somewhere in the neighborhood of $350 per sq. ft. If you want to go to the high end on your finishes, the costs will be greater. And in every remodel budget you have to have a contingency budget because you will encounter unforeseen problems.
Your wisest move would be to retain the services of a CAD expert or an actual architect to create drawings of your expansion/remodel. That gives contractors a concrete tool on which to base a bid. And it greatly simplifies the permit process.
If you would like, I can refer you to people who can provide you with these services, and contractors who can give you bids.
Alain Pinel Realtors
DRE # 01319204