A bill creating a new District of Columbia Housing Finance Agency with broad power to raise and lend money to enlarge the city's tight housing supply, notably for families with low and middle incomes, was enacted yesterday by the D.C. City Council.

Although the measure is regarded by council members as among the most important enacted in four years of local self-government, its passage came without fanfare on a unanimous voice vote.

The bill, expected to go into effect early next year if signed by the mayor, will permit the new agency to issue tax-exempt bonds to borrow funds, and to lend the proceeds at low rates to housing developers.

The agency also could become a conduit for channeling federal housing funds into local projects. It also could make funds available, through private lending institutions, to home buyers otherwise unable to obtain mortgages.

With yesterday's action, the District of Columbia will join 41 states that have created similar housing finance bodies. The measure was sponsored chiefly by Council Chairman Sterling Tucker and by Nadine P. Winter (D-Ward 6), who chairs the council's housing committee.

The D.C. version is patterned after one adopted byMassachusetts in that it requires that loans in the program must be channeled to developers who agree to set aside specific proportions of units for occupancy by families in a lower hyper and middle-income ranges. The legislation encourages a mix of income levels within each project.

For example, to get construction financing for a rental progject, a developer must reserve at least 15 percent of the unit for low-income people.

James H. Harvey, executive director of the Metropolitan Washington Planning and Housing Association, applauded the measure's passage.

"It will act as a stimulus and catalyst for other types of investments, and doesn't preclude private institutions from getting involved." Harvey said. "All I can say is - let's get it on, let's get it established."

"I've been in favor of the concept," said Thomas J. Owen. president of Perpetual Federal Savings and Loan Association, the city's largest such institution. "I'd venture that lending institutions want the city to avail itself of every possible tool to improve the housing situation," he declared.

Under the measure, the new agency will be administratively independent of the D.C. government, and the money it raises through selling of bonds or notes would not be counted against municipal debt. The bonds would be paid off from mortgage payments by borrowers.

On another housing issue at yesterday's meeting, the first lawmaking session after a summer recess, the council voted preliminary approval of a law permitting a mortgage interest rate of 11 percent for the next three years. Since 1974, until it was raised to 11 percent in June on a temporary basis the rate had been 10 percent.

The vote was 9 to 2 with David A. Clark (D-Ward 1) and Hilda Mason (Statehood-At Large) opposed.

In other matters yesterday, the council:

Enacted a bill permitting persons undergoing psychiatric treatment to inspect their own records, and to safeguard those records from access by unauthorized individuals.

Voted preliminary approval of a bill establishing expanded legal rights for retarded persons, and setting up procedures for transferring residents of the city's Forest Haven facility into community-base homes.

Approved a resolution by Marion Barry (D-At Lrge), the apparent Democratic nominee for mayor, that calls for a doubling of the annual federal payment to help pay the costs of the city government.

Postponed action until Oct. 3 on a bill that would prohibit the establishment of new diplomatic chanceries in residentially zoned neighborhoods.