What foundation system do you use for large attached porches an garage? ICF or switch to conventional for those parts?

I'm using conventional with FastFoot for the footing and CMU for the stem for porches and garages, but my frost depth is only 12" max. Not sure what advantage there would be to using ICF over conventional footing and CMU. Maybe a little benefit in the garage if insulated and heated. But you could get essentially the same effect for less cost by just installing foam board inside the foundation wall prior to backfill.

My garage wall foundation is the same as the house since I built the garage with ICF and want to be able to heat it. The ICF in the ground helps keep the heat in the garage floor slab from migrating to the cold ground outside during the winter. I have a thermal break under the garage doors. Even when the heat is off my garage usually stays 55°F or greater in the winter. It was 50 this morning but it was down to about 5 or 6°F last night outdoors.

My porch, 6' x 26', has a slab floating on the ground. It is not anchored to the house wall nor to the posts. The posts supporting the roof structure rest on columns on top of footing pads. The footings are about 3' to 4' below the slab, but that's because the slab is on fill.

To me, the key is whether the structure will be insulated. If yes, then go with ICF, if no, then go with conventional.

I would say it depends. Chances are, it might cost you the same to switch from icf to poured because the small amount of forms that need brought in vs if they were already onsite for the rest of the bsaement. Also, around here, the "form and pour" guys do not do ICF, and vice versa. It seems both sides have strong opinions either way, so they rarely do each option. That would mean, around here, you would need to have 2 subs come on site.