I have been quoted $260 / ft by a very reputable builder with nice finishes.
I have seen building permits for less.
It will vary depending on the finishes, complications in the build, proximity to water/coast, etc.
I would contact a local real estate professional to help with this answer and to save time and money.
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I doubt it would be a construction loan. You could refinance it and get money out to make the addition or you could go for a home equity loan and use the money to make the addition. In either case you need to speak to a lender, try the bank where you keep your money since they already know you.... more