It's your kitchen you do what you want! Tell him/her that you don't really like the idea and ask him how he/she can fix it. You should trust him for the most part because he/she has a degree in decorating but it all comes down that its your kitchen not theirs.

It's pretty standard to see 12" or 15", so you should choose what feels better for you.

You can use 12 in overhangs in kitchen if you use hidden support brackets like Filipe posted. I always find 15 inches is more comfortable with the overhang but it will through off the symmetry your designer created. How many people would you ideally like to sit at the island? The rule of thumb is two feet per person. You have space in your kitchen to make the island larger by shifting it towards the sink. You can then fit four people on the back side and remove the two end seats. if you would like to expand to 15in overhang, you can use the legs for support on either side or extend the side cabinetry similar to the photo below and maintain the symmetry of the island.

IMO 12” is fine I am quite tall and do not feel I need more but I would never have seating on 3 sides of an island , people sitting in my traffic pattern when cooking would make me crazy. The steel supports are the best .

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