Kitchens can be designed so that islands are incorporated into the space. Or, kitchen islands can be added "after the fact." Space, electrical access, and sometimes plumbing, are the main considerations. Let's look at space first.
Kitchen Island Design - SpaceYour kitchen island should be no more than 10% of the total square footage of your kitchen. Any more than that, and the island will overwhelm the kitchen and make it difficult to maneuver. For example, the kitchen shown in the accompanying image is 10 feet by 13 feet (130 square feet). Its island is 4 feet by 7 feet (28 square feet). So, about 10% of the kitchen is used for the island.
Kitchen islands do not have to be this big: they can be sized up or down according to the size of the kitchen. Experiment by placing a table in the space for 2-3 days, and see how it feels. However, if you are planning to have any kind of appliances (stove, oven, or a sink), bigger is definitely better.
Most people covet islands for the instant kitchen counter space they allow. But what kind of cabinet and drawer space do they give you?