Historic Tax Credit Growth and Opportunity Act of 2021 This bill increases the rehabilitation tax credit and modifies certain requirements for the credit. The bill increases the rate of the credit for qualified rehabilitation expenditures in taxable years beginning after December 31, 2019, and before January 1, 2027, after which the rate reverts to 20%. The bill increases the rate of the credit to 30% for certain small projects whose qualified rehabilitation expenditures do not exceed $2.5 million. The bill also expands the types of buildings eligible for rehabilitation by decreasing the rehabilitation threshold from 100% to 50% of project expenses. It also eliminates the basis adjustment requirement for the credit and modifies rules relating to tax-exempt use property eligible for the credit.