I'm wondering how much of a challenge it would be to add onto a completely ICF constructed house (basement, 1st floor), with the addition also including an ICF basement, first, and second floor.  If anyone has any experience with something like this, or general input, I'm curious what the advantages or (much more likely) disadvantages would be and if there is anything that could be done while building the original structure to help make the addition simpler.

Here's my thinking:
I am in the preliminary stages of purchasing land to build on, and in even the more preliminary stages of designing the house that we'll eventually build.  I really like the idea of building a small house (ICF walls - basement/first floor, SIP cathedral roof) to start out, with the intention of adding on to it further down the road (5-10 years).  The addition would be significantly larger than the original house and contain a basement, first floor, second floor.

Thanks.

Notes:
In my scenario, the entirety of basement would be unfinished.

There is really no reason you can't do this. All it entails is exposing the existing foundation, stripping foam to get access to the concrete, and putting in anchors to tie the two together before you pour, maybe even going so far as to provide some conduit stubs between the two for water, power and sanitary sewer when you add on. You would want to plan services and backfill in such a way that it didn't interfere with your future plans.

Consider doing a double row of #5 horizontal rebar about 7' above the floor levels, this way when the addition goes on you have already dealt with the lintel for most openings, or better yet, if you know where the openings will be for the future, just add both double top bars, and verticals beside the future door opening. When to concrete is cut and opened up down the road you will be safe.

Have your engineer note what/where you did this and don't lose the paperwork as it will save you hassle later on.

To save cutting out concrete, you can simply frame in a section of wall that will be removed to allow access from the first section built into the addition (works above grade - may not work in the foundation).