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.
A kitchen island is not a new concept, since they were known back in the medieval days when knights were bold. They are assuming increasing popularity in today's kitchens, particularly in larger homes where the kitchen is more than just a small cooking area compressed into an area somewhere between the dining room and the garden shed!
Pick a countertop for your island that matches your other counters or make it stand out by using a contrasting material. Also, your choice of cabinetry and decorative elements can make your island into a real quality item, similar to a fine furniture piece.
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.
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