Why do this, since even stock plans cost you at least a couple of hundred dollars each? Because the right stock plans can save you tons of time and money if you have a talented architect. Most plans are at least partly modular these days, and often you can even order prefabricated sections pegged to a specific house plan.
Islands can "float" as well. Smaller islands used primarily as an aid in cooking or baking can be fitted with wheels and moved around the kitchen as needed to aid in cooking. Whether you consider a floating island will probably depend on what you have installed on the floor now, or plan to install (a soft wood will be nicked and scratched).
Space and Design are Important : You need around 3 feet of walking space around the island unit, and you must be sure that the drawers and doors in the cabinets have room to open. If you keep to the 36 inches then you should be OK. The only dimension that must be stated is that of the counter height. Ideally it should be the same as the rest of your kitchen units. If not, then it is not a disaster, but can be disconcerting when you are working on the various surfaces at varying heights.
If you have doubts, you could always ask a professional designer to go over your kitchen space and make a recommendation or, if you want to save money, you could visit friends who have a kitchen islands and make the necessary comparison. Moreover, you can choose for your kitchen island to be in an L, U, or a G shape.