My son has a floorplan selected but wishes to use ICFs. The outside dimension is 30’ by 48’. When one build a frame home the dimensions run from outside 2x6 to outside 2x6; while a block or poured concrete structured is measured from outside to outside concrete surfaces. With an ICF structure one basically has a poured wall with 2 1/2“ of foam on the outside. The question I pose, does one layout the home measuring from the concrete portion or the foam portion? It seems logical that one would lay it out from concrete surface to the other, thus a 30 x 48 structure would become 30’-5” by 48’-5” measuring from foam surface to the other foam surface. My reasoning is that one would want as much of the heel of the truss to sit upon the concrete as possible.

Design for the ICF block you are going to use if possible. That is if you can make the house an inch or three longer or shorter so you don't have to cut as much block, it makes life a little easier.
As far as the trusses go, order them with high heels and tell the manufacture to keep them 2" shorter on each side then the outside of the ICF. After the trusses are installed, cover the high heels with 2" of foam so that they are flush with the ICF surface.

For tax purposes, they use outside dimensions, since it's easier to just pull a tape measure around the outside of the house (and they get to tax you more on so-called 'heated square feet'). As far as the trusses go, FBBP has it right. I had my trusses made to the concrete dimension, sheathed the raised heels, then 2" of foam. The sheathing adds a little more strength. My trusses are attached to pressure treated plates, which are bolted to the top of the ICF wall. I also put foam on the inside of my plates as a thermal break. One thing to consider with ICF is the thicker walls, so you may want to go a bit bigger on the outside dimensions, taking the standard ICF dimensions into consideration. Our house is 33'4" X 43'4" because of the ICF dimensions.

The outside of the foam is the building line, or dimension. Ideally make your walls the dimensions of the ICFs you are using, approximately. Add the length of the long return and the short return of the ICF corner, added together with the web spacing (6 or 8 inch), your outside to outside wall lengths should be divisible by this number. Just think it through. I say "approximately" because EPS shrinks. In eleven years I have only had one house drawn with this flexibility.....my own. In what ideally should have been a 48'6" wall (11 straights, a long return and a short return) it came out to 48' 5". Each form was 1/12th of an inch shorter than it was meant.