Prospects for congressional passage of a plan to increase home ownership opportunities for an estimated 8 million more Americans are rated "50-50" by Vondal S. Gravlee, president of the National Association of Home Builders.

Gravlee is a leader in NAHB efforts to provide longer low-range payments in the FHA graduated-payment mortgage program. He said this week that a proposal to liberalize legislation to recognize appreciation of home prices through early years of mortgage payments has a "good chance in the Senate but not so good in the House."

The program offers a 10-year graduated-payment mortgage, with initial monthly mortgage payments low enough to qualify families with incomes between $15,000 and $25,000. Buyers would have the option of keeping the initial low monthly mortgage payment for two years or five years and then haveing their payments increase 4 or 6 percent annually.

The organized home builders are promoting what is called the Home Ownership Act of 1979 in order to make more persons eligible to buy houses in a year in which the industry group in predicting that the number of new residential starts likely-to fall to 1.625 million after being at the 2-million level for the past two years.

Gravlee said he told President Carter during a recent meeting that the housing affordability crisis is serious because "if interest rates hit 11 percent nationally, approximately only 15 percent of the American families will be able to afford a median-priced (now $61,000) single house."

Gravlee explained to a press conference earlier this week that those figures are based solely on the income level necessary for a household to handle the financing costs of an average single-family home. "We are trying to make home ownership more possible for people with incomes in the range of $15,000 to $25,000," he said. " And the new mortgage program would not require a federal subsidy."

Gravlee said that the president did not indicate whether he would support the Resources Planning Act's goal of cutting 14 billion board feet of lumber from national forests during fiscal 1980. Interior secretary Cecil Andrus told the NAHB he would support that goal if the Forest Service is transferred to his department from Agriculture.

Gravlee said that almost one-third of the lumber used in U.S. home construction now is being imported from Canada and that it represented 7 percent of the U.S. trade deficit in 1977. "I urged th president to support an increase in timber production from national forests so we can meet growing lumber needs and reduce the cost of timber," the NAHB president added.

Gravlee also was asked about possible Federal Trade Commission moves to provide a program mandated and operated by th federal government to enforce new-home warranty protection for buyers in the wake of a rash of complaints about shoddy housing. He responded that the NAHB-sponsored Home Owners Warranty program is working and gaining more use-accounting for one-fourth of all new home starts last year. Asked why more builders do not adopt the HOW program in the face of the threat of federal intervention, Gravlee said: "They should see it (the need for HOW coverage to protect consumers)." CAPTION: Picture, VONDAL S. GRAVLEE . . . Home builders president