A design-based research (DBR) approach was used to investigate how a mathematics geometry study which employed the principles of universal design for learning (UDL) within a discipline-based inquiry which embedded assessment for learning impacted student learning, and teacher learning and instructional designs. Qualitative and quantitative data informed the research findings and indicated: (i) all students showed significant improvement in achievement, (ii) all students made gains in the five strands of mathematical proficiency, (iii) all students can engage with difficult mathematical ideas when they are provided with assessment for learning, (iv) the principles of UDL permit teachers to break the stranglehold of the procedural script for teaching mathematics, (v) access to technology is a critical factor in an accessible mathematics classroom, (vi) introducing UDL and assessment for learning into the mathematics classroom is a disruptive innovation, and (vii) creating accessible mathematics classrooms, consistent with UDL and assessment for learning principles and practices, requires increased teacher knowledge and support for on-going professional development.