Legislation to further liberalize the provisions of Federal Housing Administration graduated payment mortgages has been introduced in both houses of Congress.

Called the Homeownership Opportunity Act of 1979, the proposal would modify existing FHA Section 245 graduated payment mortgages by reducing down payment requirements and extending the period of below-average amortization payments while recognizing expected appreciation in the value of purchased homes.

The bill was introduced by Sen. Harrison A. Williams Jr. (D-N.J.), chairman of the Senate Banking subcommittee on housing and urban affairs, and by Rep. Les AuCoin (D-

MECHANICS OF THE LIBERALIZED GRADUATED PAYMENT MORTGAGE WERE WORKED OUT EARLIER BY VONDAL S. GRAVLEE, NEW PRESIDENT OF THE NATIONAL ASSOCIATION OF HOME BUILDERS.

IN PROPOSING THE PLAN TO THE NAHB convention in Las Vegas in January, Gravlee said it would enable an additional 8.5 million American families with incomes between $15,000 and $24,000 to become home owners.

Under one option in the proposed legislation, a buyer could obtain a $55,000 house with annual income as low as $18,000 instead of the $24,900 now required. With a down payment of $2,250 and an FHA mortgage interest rate of 9 1/2 percent, the monthly mortgage payment for the first two years would be $317, or $148 less than under a standard fixed-rate mortgate.

Mortgage payment would increase at the end of the first two years at the rate of 6 percent annually for 10 years to a predetermined level.

Under current FHA graduated-payment financing, the down payment on a $55,000 house is $4,200.