Mortgage rates have seen their peak and could decline to the 12 to 13 percent range in the late fall. But don't bank on big decreases from current levels -- bringing rates much under 15 percent -- until after mid-year.

That's the good news about mortgage money, in the opinion of some of the nation's top housing economists. The not-so-good news is that for your local bank or savings and loan association to begin offering mortgages at sharply reduced rates, it will have to begin seeing strong inflows of deposit money, and that's not happening.

On the contrary, most thrift institutions are still losing deposits -- an estimated $1.5 billion or more in net outflows for S&Ls alone during the month of April -- and can only afford token drops in rates for the immediate future.

If you're looking ahead trying to assess the mortgage market in connection with buying or selling plans for the next nine months, here are the key factors to watch:

The Carter administration's "success" in triggering a recession. Ironically, home buyers and sellers won't get a real break on rates until the economy flattens out and competition for credit eases off. If businesses keep plunging into the capital markets to finance plant expansions and inventory -- despite a prime rate without parallel in the nation's history -- money will continue to be bid out of the reach of most other borrowers.

If, on the other hand, corporations begin scaling back their borrowings, as may now be under way in earnest, the cost of money will drop. The recent small decrease in the prime rate is probably the first of a succession of such declines.

The best guess, according to Thomas Harter, economist for the Mortgage Bankers Association of America, is that the prime will be at 15 or 16 percent in late August or early September, and the U.S. economy will be well into its long-awaited recession.

Self-restraint by consumers in the world of "plastic" credit. The Federal Reserve's crackdown on credit card borrowing, consumer loans and checking account overdrafts is aimed at one of the biggest sources of inflationary steam in the economy since the mid-1970s -- easy money by the tens of billions of dollars for consumer items ranging from TV sets to motorboats.

The early results of the Fed's measures are encouraging, at least from the point of view of prospective home mortgage borrowers hoping for more capital.

Federal actions to encourage savings at thrift institutions. The recent imposition of controls on money market mutual funds is likely to begin diverting some funds back to the thrifts, which offer competitive rates as well as insurance on accounts of up to $100,000.

Another move under study by financial regulatory agencies in this political year, however, could help even more: reinstitution of the 1/4 percent rate differential on money market certificates formerly enjoyed by thrift institutions over commercial banks.

Lenders whose primary function is to provide mortgage credit are pushing hard for this competitive "leg up." When S&Ls were permitted two years ago to offer six-month, $10,000-minimum certificates with this rate differential, it enabled them to attract 54 percent of all money-market certificate deposits. The bulk went directly into mortgages.

When the federal government responded to pressure by withdrawing that advantage from banks (who don't have to put their dollars into housing), the S&Ls' share of these funds dropped sharply.

Dennis Jacobe, economist for the U.S. League of Savings Associations in Chicago, says restoration of the 1/4 point differential this year "could be extremely beneficial to anyone seeking a mortgage in 1980." The change -- essentially a political and economic call by the Carter administration -- wouldn't cost anything in terms of the federal budget, but would effectively pump new money into housing.

The international situation. If a conflict in the Middle East breaks out this spring or summer and produces a drastic reduction in oil supplies to Europe and the U.S., "all bets on interest rates are off," says Peter Treadway, chief economist of the Federal National Mortgage Association.

Any serious problems in the international credit markets -- in which oil plays a key role -- could force the U.S. economy into a steeper slide than presently anticipated. That, in turn, could push inflation beyond even current levels, and drain capital away from mortgage lenders.