Another problem you might face if you buy your kitchen island as a separate unit, and not as part of the overall kitchen, is matching it to your existing cabinets. This might not be as easy as your think, but if you purchase a solid color, such as black, white or red, then you should get away with it.
Take the opportunity to make your kitchen island mutli-functional because there's lots of options to choose from. Just about any appliance can be built into it including cook tops, under-the-counter refrigerators, sinks, dishwashers and trash compactors. Deep drawers provide a place to store bulky cookware and pullout shelves allow for more easy access.
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.
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