Build for Future Disasters Act of 2021 This bill eliminates certain National Flood Insurance Program (NFIP) rate subsidies for newly constructed property. Specifically, the chargeable rate for NFIP coverage of newly constructed property and any substantial improvements of property started on or after January 1, 2025, must not be not less than the estimated risk premium rate. The Government Accountability Office must report on the feasibility and effects of (1) eliminating by January 1, 2027, all subsidies that reduce premiums for NFIP coverage to amounts below those necessary to operate to program without a deficit, and (2) prohibiting these subsidies unless flood mitigation activities have been completed on a property.