You'll need a loop vent or a valve type vent for your drain lines but I don't see why those and the supply lines wouldn't fit inside the sink base cabinet. They should enter the bottom rear of the cabinet. I'm not seeing any need for this wall.

We are moving our plumbing to the island from the perimeter and have not had this issue. Do you have an unfinished basement for access? Since you mention trenched, I'm guessing no. Is there code pertaining to this in your area? Did you ask your GC to explain why in detail? Something is up since it's not in the architecturals.

It was Project Manager who has been telling me this. I just spoke with GC. He says it's code here in So Cal due to EQ...plumbing wall must be strapped to floor...

It's typical, at least in our area, (so cal) to have a pony wall in an island to bring the plumbing up in. Otherwise, where would you mount your hot and cold supply shut offs? And where would the waste pipe go?
You also need what's called an "island loop" to vent the sink drain line, since there is no vertical vent up a wall to the roof.

This post was edited by ctycdm on Wed, May 21, 14 at 0:15

We also have plans to bring plumbing to an island in a slab house and not one of the contractors that has come through has mentioned a pony wall. We've discussed the need for a loop vent but they've all agreed that even that doesn't require much extra room in the sink base. We're in NorCal so maybe code is different but it may be worth your while to call your city building dept. before making any unwanted design changes.....

I'm in SoCal and have a shallow pony wall in the middle of my peninsula between the cabinets. This is where the plumbing for the sink is located.

XTacie--Ask the next contractor and let us know the results. I would think if you are in the Bay Area you would have the same EQ codes, however. Perhaps not in Sacto or other locales.

No one mentioned this before to me either, but it seems to be de rigeur based on these comments...?

I designed my island with a "cavity" that would house the "loop" or the sink drain as well as electrical for my wall oven (that is mounted under the counter near the sink) and also electrical for a "pop up" outlet. I don't think "code" where you live is an issue here. Sounds like your G.C. or architect or whoever designed your island (was there a drawing with dimensions?) forgot to mention that of course there needs to be room somewhere for plumbing vent (and electrical if you have a pop up outlet.)
I'm a newbie at this stuff. Guess I was lucky that I had plumbing put in first and designed the island around the opening that would obviously be needed for the vent and electrical. Didn't realize what a smart thing this was to do. I don't see how you have any choice. I don't see how the vent could go up through the sink cabinet unless you've got a very narrow width sink.

"Otherwise, where would you mount your hot and cold supply shut offs? And where would the waste pipe go?"

ctycdm:

The drain and supply lines come from the floor into the bottom rear of the sink base cabinet. The shutoffs can be mounted to the cabinet sides. As long as electrical wire is encased in armored cable, it can be run inside cabinets too. A loop vent doesn't take up enough space to seriously alter sink size.

This wall may be required for seismic reasons, but not for mechanicals.

@ChrisInSD, The contractor that we're getting ready to hire did indeed say that a pony wall needs to be built :o(. He wants it for attaching plumbing and electrical wiring. I forwarded Trebruchet's explanation to him but so far he hasn't responded. There is a $1500 charge in the bid for the unwanted wall so the least I may do is suggest the cavity that elphaba mentions. I'll call the city to double check and let you know the outcome...