From Quora: My answer to "Will students with IEPs be able to use test accommodations on a standardized test such as the ACT, SAT, and SAT subject test?"

https://www.quora.com/Will-students-with-IEPs-be-able-to-use-test-accommodations-on-a-standardized-test-such-as-the-ACT-SAT-and-SAT-subject-test/answer/John-Linneball

Only if you apply for the accommodations through the ACT and SAT test providers (American College Testing and the College Board). Having an IEP for your normal schoolwork, without more, will NOT get you the accommodations you need. In fact, the SAT states, right on the webpage I link below, “Scores will be canceled if accommodations are used without College Board approval. [Emphasis in original]” You’ll have to provide documentation from your school, health professionals, etc. to the ACT and SAT people, several months in advance (seriously, I’d start a year before taking the test if I were you).
The information is at the SAT and ACT’s websites. The SAT information is at Students with Disabilities and the ACT information is at Accommodations. [Arnold Schwarzenegger voice] Do it now!