The Minneapolis-based McKnight Foundation has announced a $23 million grant to the Family Housing Fund to build 5,000 affordable-housing units in the Twin Cities area during the next four years.

This is the foundation's largest grant to a single organization.

The new grant means the foundation has contributed $68.7 million to the Family Housing Fund since 1980 and a total $101.7 million to affordable housing programs during the same period.

The $23 million will construct new housing or refurbish existing units in the Minneapolis-St. Paul region. The units will be used by families with annual incomes of less than $36,000, the foundation reports.

The vacancy rate for rental housing in the Twin Cities area has dropped from more than 7 percent to 1.5 percent since the early 1990s, the foundation says.

The McKnight Foundation helped launch the Family Housing Fund in 1980, with the support of the Minneapolis and Saint Paul municipal governments.

The foundation, with assets of $1.9 billion, made $77.3 million in grants last year. It was established in 1953 to fund regional, national and international human services, health, human rights, arts and scientific causes.