Good luck and I hoped this helped!
Coldwell Banker Residential Brokerage
209 Central Avenue
Westfield, NJ 07090
It would depend upon the type of addition you are considering. Adding on space that would be kitchens or baths will likely be more costly than, say, adding on a single-floor family room - even tho the square footage may be the same.
The same is true of building a 2500 sq foot house -- even if you leave the price/size of the land out of the equation, the price of the home will depend upon the specifics of the home (e.g., style - ranch vs colonial vs. bilevel; and how 'upscale' the appliances and other finishes are).
If you are interested in new construction, I can be of assistance to you as there are properties currently on the market that may match your requirements. If you have very specific needs and want a custom-built home, I can also assist you with that.
Prudential NJ Properties - Westfield Office