The nation's varied local housing markets are expected to rebound in the second half of this year -- but not quite as fast as they plummeted in the first half of 1980.

Total starts for the year are expected to be 1.2 million -- just slightly better than in 1975 but one-third less than last year.

These are findings of a survey of U.S. and major local markets pubished by Advance Mortgage Corp. An affiliate of Oppenheimer & Co., Advance is the nation's third largest mortgage banking firm.

The housing rebound will be confined to the for-sale market, according to the Advance study. Rental starts will stay low well into 1981. This sector has a new problem -- doubling up -- on top of its long-standing problems of high costs and scarce financing. For the first time in years, rental vacancies are reported to be increasing in many local markets.

"The clearest signal of the imminent upturn (in home buying)," says Advance Mortgage president Robert J. Mylod, "is the buyers' panic in the GNMA securities market. (These are securities backed by FHA-VA mortgages and guaranteed by the Treasury-funded GNMA.)

"Yields have dropped three full percentage points (300 basis points) in less than eight weeks. GNMA yields are now 11 percent (compatible with FHA mortgage yeilds of 11 1/2 percent) and could drop as low as 10 percent this summer before bottoming out.

"Institutional investors are scrabling to lock up yields above the underlying inflation rate. (This is the rate of built-in inflation, after stripping away such transitory factors as petroleum-price and mortgage-rate increases. It is now generally estimated at 8 to 9 percent.)"

In the conventional mortgage market, some lenders are now at 12 1/2 or 12 3/4 percent. By July if not sooner, the Advance report says, 12 percent rates should be general and many lenders will be at 11 percent.

Very soon, it points out, the thrift institutions will begin enjoying strong inflows in 30-month-certificate savings.

"Individuals, too," Mylod sayd "are eager to lock up high yields as they see the market tide turning and a developing, rather scary recession is another incentive to save.

There will be a lag of some months, Mylod says, before the lower mortgage rates are translated into housing demand. A 12 percent mortgage rate may appear a bargain to a homebuyer who looked at 17 percent rates a few months earlier but his tendency will be to wait and see if rates aren't going even lower.