American Housing and Economic Mobility Act of 2018 This bill addresses housing affordability and access by establishing a Local Housing Innovation grant program to support efforts to remove barriers to affordable housing; reauthorizing and increasing funding for various specified housing programs; establishing a Middle Class Housing Emergency Fund to support the construction of affordable rental-housing units; establishing a down-payment assistance program; establishing a formula grant program to assist borrowers who have negative equity in their homes; expanding the applicability, and increasing the penalties for violations, of certain requirements for financial institutions to help meet the credit needs of lower-income communities; expanding certain protections under the Fair Housing Act; and otherwise revising provisions related to housing assistance. The bill also lowers estate-tax thresholds, establishes new tax rates above those thresholds, and otherwise modifies provisions related to transfer taxes.