Buoyed by the recent sharp downturn in mortgage rates, housing experts yesterday predicted a modest comeback in home purchases this summer and a slight upturn in home construction by the end of this year.

But they don't expect the current slump in the total housing market to end until there is evidence of greater consumer confidence in the total economy and an acceptance of mortgage rates that aren't expected to fall below 13 percent this year.

While not disguising their surprise at the rapid downturn in lending rates -- particularly those for federal borrowing in the past eight weeks -- a convocation of housing and finance professionals retreated from instant optimism.

Panelists assembled by housing industry economist Michael Sumichrast at the National Housing Center for a semiannual residential construction forecast presented these views:

"I'm predicting 1980 starts will be between 1.2 and 1.3 million," said Lawrence Simons, Housing and Urban Development assistant secretary for housing. Simons reminded more than 200 housing professionals that his previous forecasts for starts also were optimistic and on target in 1979 (1.75 million) and 1978 (2 million), when other forecasters had predicted fewer starts. "Buyers are waiting in the wings for rates to drop," he said.

In an interview, Simons didn't discount speculation that the FHA-VA rate ceiling might be lowered shortly from 13 percent to 12 percent because loans are being made under the mandated ceiling. He said investors in mortgage securities are scurrying to lock current high yields in anticipation of falling rates due to recent lack of demand.

On the other hand, Sumichrast said housing cannot possibly recover significantly this year and that buyers need even lower mortgage rates.

While describing buyer cancellations of contracts to buy new houses as "enormous," Sumichrast added at another point that "you don't buy a new house when your neighbor has been fired from his job."

Jay Janis, chairman of the Federal Home Loan Bank Board and a former builder and No. 2 HUD official, posed the conflict between a recession and a housing recovery. All panelists agreed that nation is in a recession. Janis said: "It's just a question of how deep it will be." He predicted starts of only 1 million residential units this year and the necessity for mortgage rates to fall below 13 percent to support a solid housing recovery.

"But I'm confident of a strong housing year in 1981 and a good market through the 1980s" because of new household formations, Janis said. He also expressed confidence that the public will accept renegotiable-rate mortgages that thrift institutions will be offering in a few weeks.

Kenneth Kerin, economist for the National Association of Realtors, said the resale housing market has bottomed at a rate of about 2.5 million annually, "but resales usually lead the recovery by several months, and the market should be stronger this summer."

He forecast an interest rate level of 13.5 percent by the end of the year and said resale prices, although rising slower, are still 10 percent above those a year ago.

Several panelists stressed the affordability of new housing, noting that a resurgence of demand could drive prices up again and then also cause another upward swing in mortgage rates. Fears of a big recession were negated by Jeffrey Green, a forecaster, who added that this one will be "bad enough."

Peter Treadway, economist for the Federal National Mortgage Association, insisted that despite high borrowing rates housing is still a good investment because of tax advantages. He predicted more than 1.1 million starts this year. That forecast struck a mean for all the panelists.