Now think about it, would you drive your car to a distant location without using GPS or a map to get there? Would you want your Doctor operating on you without a game plan? A good, detailed set of plans and specifications are paramount to a successful construction project. They will not only save time and mistakes, but the price that you "invest" in a set of plans will come back ten fold.
A good set of plans should have details that answer all of the questions that may come up. Some of the elements in the plans should be a floor plan, structural plans (if there are walls being moved or built), interior elevations of the walls, electric plan, window and door schedules (lists of types and sizes of doors and windows), a finish schedule (the materials that will be used on the floors, base boards, walls, etc.), and any other elements of the project that can be listed or drawn.
Anything that is not on the plans, should be written on a specifications sheet or book. These may include tile type, paint manufacturer selection, colors for the paint, door hardware, light fixture types, plug and switch types, plumbing fixtures (make and model number and color) and interior wall finishes. Well, you get the idea. The more the detail, then the more likely there will not be a misunderstanding during the project.