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.
Countertop material for the island doesn't have to match the rest of your kitchen countertops as long as it is harmonious with the room's overall design. You may want to splurge on solid surfacing here, for example, and use laminate on the other countertops. A butcher-block countertop is ideal for chopping, while granite or marble works well for baking purposes and for rolling pastry dough.
Kitchen islands work best in larger L, U or G-shape kitchens. If the kitchen is too small, the kitchen island will become an obstruction and hinder easy movement. The best custom kitchen islands for small to midsize kitchens are a portable butcher block or kitchen cart for food prep or extra storage.
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.
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