At the design stage — before framing, before tile, before any of the decisions that make retrofit expensive. Grab bar blocking: $50–$150 during construction vs. $800–$2,500 as a retrofit. Zero-step entry: design default vs. $5,000–$20,000 retrofit. 36-inch doorways: no cost difference at framing vs. $1,500–$4,000 retrofit per door. Elevator hoistway: structural provision vs. $20,000–$60,000+ to core through existing structure. These features add 1–3% to construction cost when designed in and become 10–20× more expensive as retrofits. They also add real estate value: a home designed for the full range of human ability is marketable to a broader pool of buyers at every price point.