Te District of Columbia plans to spend nearly half a million dollars to form a law-interest-loan fund for owners of low-income rental housing in three southeast and far southeast neighborhoods.

Landlords of about 150 apartments, duplexes and fourplexes in Bellview, Garfield Heights/Knox Hill and Naylor Gardens will use the money to rapair and spruce up their units. Rents can be expected to increase but not to levels the present tenants cannot afford, said Alice Vetter, executive director of MUSCLE -- ministries United to Support Community Life Endeavors. The nonprofit community housing organization will direct the rehabilitation program.

Median rents for unfurnished, one-badroom apartments now are $225 a month in Bellview and Naylor Gardens, and $185 in Garfield Heights/Knox Hill, Vetter said.

The District's contribution of $450,000 to the loan fund will come from the federal Community Development Block Grant money the city will receive for the 1983 fiscal year beginning Oct. 1. MUSCLE and city housing officials expect three times this amount to be added to the fund in the from of market-rate loans from private lenders. The mix of money should result in loans to property owners at interest well below the market rates, according to the city housing department.

In addition, the Department of Housing and Urban Development will provide rental assistance for about 50 tenants. The money will come from the Section 8 program, under which low-income residents pay only one-quarter of their income for rent, with federal funds making up the rest.

The housing to be rehabilitated, all in buildings with fewer than 30 units, will be chosen by the end of October after meetings of MUSCLE workers with property owners, Vetter said. Planning will occupy the winter months, with work scheduled to start next March and be completed by the autumn.

In picking the units to receive the loans. MUSCLE will look for cordial relationships between landlords and tenants so they cab work together to set the amounts of rent increases. Vetter said. She said the organization has found that "a kind upgrading the building... is what's needed to keep low-income housing" in the city.

Private contributions to the loan fund have not been obtained yet. But Vetter said that MUSCLE, which was formed in 1978 to work toward halting displacement of low-income tenants in the city, has had "a very good record" in inducing private lenders to participate in its housing programs.