Sallie Mae, the student loan company, announced yesterday that it is awarding $28 million to help charter schools in the District acquire or lease facilities and expand their student enrollment.

The announcement is one of the largest financial commitments to date to help the space-strapped charter schools, many of which are housed in overcrowded and makeshift facilities. Last month, Mayor Anthony A. Williams (D) and Sen. Mary Landrieu (D-La.), chairman of the Senate Appropriations subcommittee on the District, kicked off CityBuild, a $12 million federally funded effort to create or expand charter schools in selected neighborhoods.

The District's publicly funded charter schools, authorized by a 1995 federal law, are generally free from the rules governing regular public schools. Although there is little evidence that they have done a better job on the whole than regular schools in raising student performance, charter schools are popular with parents and have grown at a rapid pace.

According to Friends of Choice in Urban Schools, a D.C. group that promotes charters, the city's charter schools have an enrollment of 14,007 students this school year, an increase of more than 20 percent from last year. The group reported that 18 percent of the District's public school students are enrolled in charter schools. The figures are based on unaudited reports from individual schools.

Sallie Mae's chief executive, Albert L. Lord, has been an outspoken proponent of charters and has called for the company to help create 10,000 new slots in the District's charter schools. "Unfortunately, many students lack the critical K-12 educational foundation," Lord said in a statement. "This is certainly true in the District of Columbia. Charter schools provide the skills necessary for students to enter college and succeed."

The Sallie Mae Fund, the Reston-based company's charitable arm, is providing the $28 million to Building Hope, a nonprofit organization created by Sallie Mae in November to provide D.C. charter schools with loan guarantees, low-interest loans, grants for technical assistance and help in such areas as site selection and lease and purchase negotiations. S. Joseph Bruno, president of Building Hope, joined the organization after a career in health care and financial services. Congress has appropriated $2 million to support Building Hope.

A Sallie Mae spokeswoman said the funding has been used to help four charter schools create 700 slots for the new school year. The schools are the Cesar Chavez Public Charter Schools for Public Policy, which has added 220 seats; the Community Academy Public Charter School, which has added 300; the D.C. Preparatory Academy, which has added 50; and the E. L. Haynes Public Charter School, which recently opened with 130 students.

Building Hope selects schools to assist based on the leadership and experience of their administrators, their financial integrity and their plans for expansion, said Kathleen deLaski, a Sallie Mae spokeswoman.

Emily K. Lawson, the founder of the D.C. Preparatory Academy in Northeast, which has 150 students in the fourth through sixth grades, said that $500,000 in loan guarantees from Sallie Mae was a critical part of the school's $6.1 million effort to buy and renovate a former warehouse as a permanent site.

"As a startup, it's very difficult to assemble the financing necessary to obtain a permanent home," Lawson said. "Sallie Mae filled an important gap in our financing plan that enabled us to renovate and move into this beautiful building."