Officials of the University of the District of Columbia have told the District government and university trustees that acquiring Antioch Law School would cost far more than was estimated when the board of trustees endorsed a takeover of the troubled law school in late March.

The new figure of at least $5 million in city funds for the first 15 months after a takeover has prompted Joseph Webb, the trustee who proposed the board's takeover resolution, to change his mind. At the board meeting the trustees were told that the full cost of acquiring the 362-student school would be $2 million.

Late yesterday, Mayor Marion Barry did not include funds for the Antioch takeover in his supplemental budget request for the 1986 fiscal year, which ends Sept. 30. But city budget Director Richard Siegel said funds may be added later while the request is under review by the D.C. City Council.

A UDC planning group, headed by Acting President Claude A. Ford, said last week that $1.3 million would be needed in the supplemental budget so the university could take over the law school on July 1. Earlier, the supplement had been estimated at $700,000. In addition, UDC officials said $3.7 million to $4 million would be needed during the 1987 fiscal year, compared to the $1.3 million that the council previously approved.

In later years, the law school would require an annual city subsidy of about $2 million, the staff report said. It said a new permanent facility for the school would cost $7 million to $8 million.

Webb said he would no longer support a UDC takeover of the law school because "I now realize that we were given less than accurate information before." He said he feared that the law school might drain funds from existing UDC programs, which already face budget reductions next year.

But another trustee, Peter Edelman, said he would support the higher funding when the board meets to consider a formal takeover plan on May 29. "I think these budget figures are appropriate," he said.

Board Chairman N. Joyce Payne said the American Bar Association indicated that UDC and the city must make a firm commitment to remedy all of the law school's deficiencies before it can approve a transfer.

The school at 2633 16th St. NW is affiliated with Antioch University of Yellow Springs, Ohio. Last fall, Antioch trustees threatened to close the school, saying they were no longer able to support it.