D.C. officials have reported that the apparent misappropriation of patients' personal funds at Forest Haven, the city's facility for the mentally retarded, is believed to be more extensive than was first believed.

In a report to the mayor's office, D.C. Inspector General Joyce Baylock estimated that a total of about $70,000 withdrawn from bank accounts belonging to patients at the 400-bed facility in Laurel, Md., cannot be accounted for now. Previously, the city had estimated that only about $40,000 was believed to be involved.

Lemuel L. Taylor, acting chief of the Field Accounting Office at Forest Haven -- whose job included administration of the funds -- was charged with theft and misappropriation of funds in a 16-count indictment returned in February by a Baltimore grand jury.

Among the findings in Blaylock's report is that $1,550 was withdrawn from a Forest Haven internal account in May 1980 to pay the cost of a trip by a group of patients to Busch Gardens amusement park, but the trip never took place. The money, Blaylock found, was never returned.

Blaylock's office, which is charged with investigating possible malfeasance by D.C. officials, is assisting the U.S. attorney's office in its probe of the missing funds, the report states.

The report covers the period between Oct. 1, 1978, and Sept. 30, 1980. On the latter date, the report states, patients at Forest Haven had about $1 million on deposit in two banks here and about $550,000 in a central account credited to individual patients and from which withdrawals could be made to meet their personal expenses.

Blaylock's staff found "significant weaknesses in the administration of personal funds," including "inadequate accounting records and procedures, noncompliance with District regulations and policies, inadequate separation of funds."

"Our review of the accounting records and procedures . . . disclosed an almost complete lack of control and accountability over residents' funds on deposit in savings accounts," Blaylock states.

During the period covered, there were 311 withdrawals from the patients' accounts totaling $124,843. Of that total, only $58,501 could be accounted for, leaving $66,342 that was not recorded in the central Forest Haven account from which the money was supposed to be disbursed.

In addition, about $3,500 was found to be missing from the accounts of patients who had recently died, the report states, bringing the total of funds that cannot be accounted for to about $70,000.

Blaylock recommended that James E. Buford, director of the city's Department of Human Services, which oversees Forest Haven, institute a number of new controls over patients' bank accounts.

She proposed that individual financial plans be developed for each patient, to help them make the most interest on their money, and that cash gifts to patients and to the institution be more closely monitored.

Blaylock said that he would institute the new controls, adding he had already done so in some cases.

He also asserted that he intended to have the city reimburse patients for any money that cannot be accounted for.