Hi. You didn't mention whether or not you are in Falls Church City or the part of Fairfax County with a Falls Church postal address. In Fairfax County, the code is on-line. In Falls Church City, you would do well to visit the permit office, which, as far as I remember, is located on the lower level of the City Hall, and asking there for builders with experience in doing what are called "pop-tops".
As a retired builder, I can say this: I have never seen a 1950's rambler built in Arlington County that could not be made to support a second story. A lot of "pop-tops" are being built in Vienna, too. That's just a simple engineering problem. Even foundation work, with modern engineered materials and techniques, is not that big a deal.The design,in and of itself, represents a considerable investment of time and money. I don't think that a seller in this heating-up market will be willing to take his property off the market for the two months it would take to find a builder and have his architects and structural engineer prepare a design. Furthermore, under the new statewide building code, building permits expire after, I think, two years, if work has not been started.
I would be much more concerned with saving the money to do the "pop-top" right, which, currently, is running at least $250,000.00. You will want, at a minimum, to raise the ceiling joists up so that you have a high ceiling. I think you should buy the house and look at a lot of "pop-tops" in Arlington and Vienna. A lot of the ones in Vienna, especially, are really attractive. Your Realtor should be able to put you in touch with a reputable builder, or, if not, you can contact me for more information.