you'll need to check the specifications on your cooktop/vent pairing. my island is 28 3/4" deep to hold the 36" thermador cooktop and downdraft; that includes a decorative back panel. it's def. full to the back and takes up the back half of the space in front of the downdraft, this is with the inline fan in the basement too. (which makes it quieter in the kitchen.)

You'll find that information in the cooktop and vent specs. You'll need at least 2" behind the cooktop and an empty space below the cooktop. Frameless cabinets will give you a bit more room. If possible I would also install an additional in line or exterior blower because the in cabinet blower will certainly not be adequate.

A kitchen layout is always better if the cooktop can be relocated from an island.

if my cooktop wasnt in my island, i wouldnt have a cooktop. so never say never.

I agree with Sophie Wheeler's comments about this and she has designed many more kitchens than I have.

yes but the truth is in the last 2 words. 'most people'.

if one has a very short L kitchen, facing door, door, door, window, stairs down, stairs up, and then opening to back hall..... something goes on the island. can't be the fridge, that leaves the sink or cooktop. sink is in turmoil all day long, cooktop pristine except while cooking dinner... ding ding ding winner for the island is the cooktop. : )