I am planning on adding an addition in the spring and wasn't sure what the norm was on needing an engineer.  Foundation will be sitting mostly on ledge, so the material the house will sit on is not a structural issue.  I originally was planning stick built, and my town inspector just said to just draw something up.  Concrete homes are not popular in my area, so the inspector will probably just be in a state of confusion

My city requires engineered plans for anything non-standard wood frame. On the plus-side, you can do some pretty wild structures with details like long cantilevered overhangs with concrete so the engineering fee allows for cool designs...

Most ICF manufacturers have tables to address engineering questions, these tables are for 'standard' conditions. If your conditions fall outside this area engineering will be required...at the same time this also holds true for 'standard' construction methods.

Some municipalities will require an independent engineer review anyways regardless of site conditions and how 'standard' they may be...it all depends on how lax your municipality is on these items. I've worked basic simple jobs in big areas that no if, and or buts about it...needed an engineer and other areas with complex designs pushing the envelope that the building department just signed off and said go for it.

Welcome to the unknown of what we all get to deal with

A good place to start is with the HUD prescriptive Method for Insulating Concrete Forms in Residential Construction

http://www.huduser.org/Publications/PDF/icf_2ed.pdf

To the letter of the code, pretty much all ICF will need an engineer. If a municipality has literally adopted the prescriptive ICF methods, you may not, but only if it fits within the (strict) requirements of what the prescriptive parameters are, which is pretty much a 2-story plus pasement rectangular house.
But many municipalities don't bother with such silly things as building code engineering requirements.