When building a conventional wood platform floor over an ICF basement is it better for the treated sill plate to be wide enough to extend out over the ICF foam or should it merely be wide enough to rest on the concrete core?  

It seems like the sill plate should only need to sit on the concrete core of the ICF and the area between the rim joist and the ICF foam should be filled with suitable insulation. The extra width to extend out over the foam would do nothing except cost more and conduct heat/cold better.

The sill plate only needs to extend from the exterior foam to the interior face of the concrete. There's no use for the plate coming to flush on the inside and its width will cost more. Here's the flip side. If your a DIY and are renting an alignment system that will rely on the sill plate to be purchased to walk on as your scaffolding, you will probably feel more comfortable on a 2" x 10". Just a note: The 2003 IRC doesn't require the sill plate to be treated unless it is within 6" of grade in most regions.

Also, you may consider holding your plate in 1" (2" x 4" walls above) or 2" (2" x 6" walls above) and decking this foundation short of the exterior face. Then build your upper walls to hang the floor by the dimensions above. Place either thickness above of foam sheets on your rim/bond joist and fill all gaps with spray foam. Next when you sheet the rim/bond area with ext. plywood/sheeting place the sheeting over the foam board and fasten with long fasteners. Makes for less air infiltration especially if you spray foam as you put on the bond rips.

With all methods you need to put in crush blocking between the rim and the first joist that parallel any ICF walls.

I have tried all scenarios and have found they all have pro & cons.

My favorite method is to recess the sill plate into the wall by 1 1/2 “ using a simple screen board “jig”.
A 2x6 fits perfectly into the recess for a 6” wall (using the ICF brand I use).
This eliminates the need for any sill seal foam and helps cut down on air infiltration.

I have tried all scenarios and have found they all have pro & cons.

My favorite method is to recess the sill plate into the wall by 1 1/2 “ using a simple screen board “jig”.
A 2x6 fits perfectly into the recess for a 6” wall (using the ICF brand I use).
This eliminates the need for any sill seal foam and helps cut down on air infiltration.

We do the same on a above grade homes but don't utilize this method on a basement.

Dave