U.S. housing starts, which edged over the 2 million mark last year despite earlier forecasts of a downturn, now are expected to range between 1.5 and 1.8 million this year, according to a diverse panel of housing experts.

Most of those experts put the figure at 1.7 million new dwellings started this year, of which approximately 70 percent will be single-family or town houses.

Among a panel of housing and finance professionals who gathered recently for a residential forecast conference at the National Association of Home Builders, the lone federal representative was the most optimistic with a forecast of 1.7 to 1.75 million starts this year. Economist Leonard Santow of J. Jenry Schroder Bank & Trust Co., Chicago, was the most pessimistic with a projection of 1.5 to 1.6 million starts.

Jay Janis, a former builder who is undersecretary of the Department of Housing and Urban Development, insisted that starts will total at least 1.7 million this year and move to the range of 1.8 million in 1980. That rebound would coincide with the next presidential election.

The official NAHB forecast, as projected by chief economist Michael Sumichrast, is for 1.63 million starts this year. Sumichrast admitted that the 1978 NAHB forecast was well off the mark. The downturn NAHB predicted was advoided because of the introduction of mortgage money certificates that held lendable money in savings and loan institutions last year.

However, the long-range forecasts for new housing, based on a doubling of the rate of new household formations in the 1980s, are upbeat. NAHB projects that starts will be 1.7 million in 1980, 1.8 million in 1981, 1.9 million in 1982 and 2.1 million in 1983. The South is expected to be the major market in that upturn.

Nonetheless, on a short-term basis, housing is headed into a "shallow decline this year," according to Saul Klaman, president of the National Association of Mutual Savings Banks. Like Sumichrast, Janis and Santow, he expects a modest rebound in 1980.

In answer to a question, Janis said the Carter administration has no policy to slow housing as a means of combating inflation. Rather, the availability of mortgage money and the high rates of interest are considered factors in slightly cooling a housing market that has been exceptionally torrid for more than two years.

In other far-ranging discussions of housing related matters, the conference heard:

Ralph Johnson, president of the NAHB Research Foundation, blame the increase in energy cost for adding at least $400 to the cost of each new house built this year. He also estimated that up to 2,000 gallons of fuel are used in building the average house.

Colin Carter, a vice president of Chase Manhattan Bank, estimate the average U.S. family spends 10 percent of total income on energy, as opposed to 3.5 percent in 1972.

Clark Bullock of the Department of Energy say that energy conservation performance should be rewarded and more emphasis put on energy conservation in homes rather than on production of energy used in homes.

Elsewhere on the housing scene, a spokesman for the National Association of Realtors recently told a Senate subcommittee that the nation faces a housing shortage, with production falling blow the rate of 2 million new units a year. CAPTION: Chart, no caption, By Alice Kresse-The Washington Post