Accommodations play a key role in enabling individuals with disabilities to participate in the National Assessment of Educational Progress (NAEP) and other large-scale assessments. However, it can be difficult to know how accommodations affect the validity of results, thus making it difficult to determine which accommodations should be allowed. This study describes recent extension of evidence-centered assessment design (ECD) for reasoning about the impact of accommodations and other accessibility features (e.g., universal design features) on the validity of assessment results, using examples from NAEP reading and mathematics. The study found that the ECD-based techniques were useful in analyzing the effects of accommodations and other accessibility features on validity. Such design capabilities may increase assessment designers’ capacity to employ accessibility features without undermining validity.