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 to 30% for small projects (rehabilitation expenditures not exceeding $3.75 million) and caps the credit for such projects at $750,000 for all taxable years. 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 the eligibility of tax-exempt use property for the credit.