How to build a kitchen island?

the rule of thumb is 36" of clearance.

I agree about the 36", however, remember that's a minimum. One trick I've found to be very effective for measuring stuff like this, is to put paint tape down where you want it to go. That way you can adjust without building anything. You don't want everyone bumping into it!

Determine your clearance zone. When clients ask if they have room for an island, we designers must consider factors such as how many people live in the house and how they use the space. But first and foremost, we need to know the size of the room.

In a rectangular room that’s 16½ feet (5 meters) wide and 19¾ feet (6 meters) long, such as this one, the main run of cabinets would be positioned along one of the walls. The depth of the cabinets from the back wall to the front of the cabinets will measure about 25 inches (650 millimeters). Within the design, you should leave a gap between the countertop edge on the back run of cabinets and the island’s countertop edge. This space between the two working areas forms part of the island’s clearance zone, which is the space surrounding the island. The ideal distance for the clearance zone is about 3 feet (1 meter). This is best for enabling free and safe movement around the island and throughout the kitchen.

Consider safety. A safe distance for your kitchen island also includes the right amount of clearance between the island and opposing cabinets and appliances, so that all doors, drawers, ovens and dishwashers can be opened safely and without obstruction. Potentially the most dangerous item is always the dishwasher door. These doors open downward, so if you walk past, you risk tripping, falling and hurting yourself or others — particularly dangerous if you’re carrying knives or hot food. As such, it’s best to leave more space. The minimum distance you should allow between two fully extended drawers on opposing runs is about 3 feet (900 millimeters).