A second kitchen sink can be included on the kitchen island. Use a sink that is deep enough for washing large pots and pans, and consider equipping the kitchen island with a trash compactor, garbage disposal, recycle bin and even a dishwasher. Cleanup will be much easier when these appliances are close by the sink.
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.
Plan the height of your cabinet appropriately. Since most islands will have at least two stools or chairs pulled up to the surface, you may want to think about the seating or stools first. There are two kinds of bar stools with one being much higher than the other. Unless the island will be used primarily for family eating, there really are no hard and fast rules on height. It does not have to match the height of the kitchen cabinet, but often does. If it is not going to match it should be taller in most cases.
Maximize natural light by having windows and skylights, and keep kitchen wall surfaces light in color to reflect daylight. Custom kitchen islands work great by using pendant or recessed fixtures to direct light onto the kitchen island and other work areas. Electrical codes will likely require that electrical outlets be located on the sides of fixed kitchen islands, not on the top, to prevent electrical shock.
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