Revitalizing Economies, Housing, And Businesses Act of 2021 or the REHAB Act of 2021 This bill modifies provisions relating to the rehabilitation tax credit. Specifically, the bill allows an increased 15% credit for non-historic buildings placed in service 50 years prior to the current calendar year. It also allows the credit for buildings within a half-mile of public transportation facilities, for residential lodging purposes, and for expenses related to building expansion on the same block as a qualified rehabilitation building. The bill allows a 25% credit for rent-restricted housing units and for expenses related to public infrastructure projects required by state or local governments as a condition of project completion.