our average charge for something like this would be $115 per square foot without bathroom & it's all depend on square feet added to determine all the discount. We offer free in-home consulation.
Kingkong Construction: 800-801-4282
Counter-intuitively, there is even a danger that adding square footage could subtract from the value.
I provide a link below to a web site devoted to these mid century moderns.
To ensure you are not overbuilding for the area, have a Real Estate Professional, who works closely with a remodeling contractor, evaluate your plans. I find that is a problem, as most Real Estate Professionals don't have such a parternship. Find one who does, and see if they will provide a consultation regarding your plans.