“Localities rather than the federal government must have the final say in zoning laws and regulations, and a conservative Administration should oppose any efforts to weaken single-family zoning” (General Welfare, Dept. of Housing, Page 543)

In other words, local governments, not the federal government, should make the important decisions about zoning laws. And if a conservative Administration is in charge, they should stand against any attempts to make single-family zoning less strict.
Historically local and federal governments have butt heads over exclusionary zoning. Exclusionary zoning restricts the type/size of building that can be built, and the policies being proposed here have allowed segregation to persist and made affordable housing more difficult to build. For a stark example of how this can play out in the worst way, read the stories of the folks living in Louisiana’s Cancer Alley. One of the women fighting the unfair zoning laws imposed by localities in Louisiana won a TIME100 award for her efforts.