Hi Plan2add- I am a Mass. licensed builder, as well as a mortgage rep, and I always hate to give $/SF numbers- because there are many variables. But, I can give you some ballparks- assuming that you'll need a full foundation and all that. Also assuming a bathroom and typical features.
Depends on the size of the addition, and also how desperate the GC might be to do it.
You're looking at between $80- $130/ SF of gross living area.
Feel free to contact me with anything else. Thanks,