Yours is a question for a contractor. The City of Everett building code will have some impact on building costs. Is the garage able to support a 2nd story? My guess is, you may need to reinforce the foundation. In terms of construction costs, a lot depends on you. Will there be a half bath, or a full bath? Are you planning a full kitchen or just a kitchenette? I think its safe to assume a range of $75 to $125 per square foot. I presume there are several local contractors that are willing to give you a bid, at no cost to you.