Per Section 1014, the basis is the fair market value on the date of death (August of 2021 - assuming the house was part of the decedent's estate at death and an alternate valuation cannot be used, both of which would be unusual).
See https://www.law.cornell.edu/uscode/text/26/1014
Note that improvements (also in the form of remediation) can be added to the basis of the property.
