What kind of support does a 15 inch overhang on an island require?

[X_Digger]

6. Sounds like a good frame.

If the frame is a single knee wall around the cabinets, you might want to add a knee wall behind the cabinets, which will take away from the backside, but will give some support to the overhang, and create a cantilever.

e.g.





Now, depending on what material you're using for the countertop, you could notch the top plates and use a half-lap with lag screws to build a supporting frame, and support 15" easily.