This is a land use issue, but most real estate attorneys should be able to help you. Alternatively, you should be able to consult the planners at the city to find out what you need. In either case, you will likely need help from a local real estate/land use lawyer. This issue involves looking at the local zoning ordinances. I assume from your scenario that the garage services the house on the other lot. If so, you may need to build a garage on the lot you keep or get some sort of variance to permit the existing use if it is nonconforming. I am guessing that local ordinances require some sort of garage or car port. Check with the planning department and then consult a lawyer.
Talk to the City planning department and they can inform you what regulations in your area will require. If they are not helpful consider hiring an land use attorney.