I just purchased a house constructed using ICFs. I'd like to turn an existing window into a door (same width as window), and a new window about 3' away. Although I understand how ICF walls work, I have no clue as to how headers are integrated into the system and how/if a header can be retrofitted into the wall. Thanks!

Current window opening should have the rebar above and sides to support a door addition...Adding a new window should include engineering like wall thickness, current rebar in wall, new "header" height. When originally doing the icf there is supposed to be extra rebar above and on the sides of each opening. So when you add a window that rebar will not be there. Adding steel around the opening may be an option but an engineer will know what is needed. Where are you located?

Oregon coast

Ask a local engineer. The rebar required above any openings is dictated by opening size. lintel depth, and floor/roof load imposed above (among other considerations). It is entirely possible that no additional reinforcing is required, but a local P.Eng can answer that for sure.

Cutting windows in an existing ICF house can be quite a task, but it's definitely doable with the right approach. Make sure you have a good understanding of the structural implications and proper tools for cutting through the concrete and insulation. I'd recommend consulting with a professional to ensure it’s done safely and efficiently. How big are the windows you're planning to add?