A federal mortgage interest subsidy program will be broadened to enable 30,000 to 40,000 low-to-moderate-income families to buy new homes this year, Housing and Urban Development Secretary Patricia Roberts Harris announced here this week.

The revised Federal Housing Administration program, as Harris described it to the opening session of a convention of home builders, is designed to benefit "the builder, the owner and the community."

The program, Section 235, is seen as a means of helping families buy homes in a year when the housing market is expected to decline slightly because of higher mortgage interest rates and tighter availability of long-term credit.

One veteran housing attorney estimated that a Washington family of four with an income as high as $18,000, for example, might be eligible under the program for a home costing about $40,000.

The program's interest fate is being reduced from 5 percent to 4 percent for the home buyer, with the federal government paying the difference between 4 percent and the current FHA rate of 8 1/2 percent. Also, the down payment for Section 235 buyers will be reduced to the statutory minimum of 3 per cent.

Harris said that congressional authorization has been obtained to raise the mortgage limits to $32,000 generally, to $38,000 for homes for large families or in high-cost areas and to $44,000 for large families in high-cost areas such as Washington.

Family size and family income are factors in determining eligibility for Section 235 subsidy financing. Family income may not exceed 95 per cent of the median income in an area.

In a speech before the National Association of Home Builders, the HUD secretary stressed a commitment to make this program work in all parts of the country, urban and suburban, Sun Belt and Frost Belt."

The FHA 235 program, one of the casualties in the federal housing subsudy moratorium five years ago during the Nxon administration, was resurrected and revised by the Ford administration.

At a press conference Harris said the Section 235 subsidy program will apply to new houses that might be extended to include some resale housing, as did the program earlier. Harris estimated that the program might cost the federal government about $1,500 a unit annually.