Citing "the persistence of inflation and the unfortunate likelihood that it may get worse before it gets better," a leading housing finance expert forecast this week that there would be no significant interest rate declines next year.

As a result, said Federal National Mortgage Association executive officer Rober Bennett, housing and mortgage activity is headed for a year-long decline.

But his boss, Fannie Mae Chairman Oakley Hunter, said that despite the gloomy interest rate forecast, a growing and changing population assures a strong U.S. housing market for the next decade. He called 10 percent mortgage money - fast becoming the national average - a "bargain" compared with 12 and 14 percent annual appreciation in home values and said that 1978 will be a record year for housing.

Addressing securities analysts in New York, Bennett said the current rate of housing starts - 2.1 million new units on an annual basis - should decline to 1.7 million rate by early next year.

"However, the overall decline should be milder and shorter than previous housing cycles, with total starts of 1.7 million in 1979 versus 1.9 million in 1978," Bennett said in prepared remarks.

Housing activity will be depressed both by rising interest rates and "the disruptive effect" of mortgage rate usury celings in many states, he said. Bennett's company, known popularly as Fannie Mae, provides a secondary market for home mortgages and thereby pumps additional housing funds into the market, buying mortgages from orginators.

Bennett said Fannie Mae economists do not foresee a recession, "at least not in the housing industry. Our forecast, and also our hope, is that the economy will have a soft landing in 1979, as the gross national product slows in response to the higher interest rates."

He emphasized the word "hope" in a subsequent passage: "We did not anticipate the sudden resumption of the interest rate spiral after the flattening in the third quarter. Nor for that matter did we anticipate the extraordinary volume of mortgage purchases we are experiencing this year."

Fannie Mae Chairman Oakley Hunter told the same gathering that his corporation made commitments to purchase mortgages totaling $14.7 billion in the first nine months of 1978, with real purchases at $9.4 billion.

The Fannie Mae outlook followed by several days the decisions by leading savings and loan associations in California to adopt a 10 1/4 percent mortgage loan rate, the highest level since the end of 1974. Many Washington area S&Ls have been quoting 10 percent as the going mortgage rate.

Mortgage money virtually has dried up in Maryland, which has a 10 percent ceiling. In the District, the City Council is working on legislation to make permanent a temporary 11 percent ceiling, and many S&Ls are hedging mortgage commitments to passage of the extension.

There is no ceiling in Virginia, where mortgage money remains available - but at about 10 percent.