Fixed-rate mortgages are heading for a comfortable 12 1/2 percent in many parts of the country, but that shouldn't lull buyers and sellers into forgetting the financing lessons of the last five years.

Probably the most important lesson has been: Don't automatically assume that your best or only financing option is a traditional, conventional, fixed-rate mortgage.

Thousands of consumers in the real estate market this summer -- especially newcomers and first-timers -- could benefit by knowing a little about some of the cost-cutting mortgage alternatives that kept real estate afloat during the darkest days of 1981-82.

What worked well when rates went through the roof at 17 percent also can work well in today's more tranquil 12 1/2 percent environment. It all depends on how you use the problem-solving tools available. Although it might surprise many first-time purchasers in 1985, one of every six homes financed in the United States so far this year has involved the assumption of an existing, lower-rate mortgage.

An assumption means stepping into someone else's shoes. You take over the loan payments on the house, the benefits of rates and terms from an earlier era, and become the borrower in the eyes of the bank. Assumable loans not only are more common in 1985 than many consumers are aware, but they also have tremendous advantages to both parties in a transaction.

For the alert seller, offering a house with an existing, below-market-rate loan attached can make the property more salable more quickly, at a higher price. For the alert buyer, taking over a mortgage can produce significantly lower-cost money -- and more flexible terms -- than any institutional lender could provide.

To make the most of an assumption on the purchase of a resale home, new buyers should look to the seller to provide a key, additional part of the financing: a so-called "purchase-money" loan, usually in the form of a second mortgage or deed of trust.

Let's say that you, as a first-time buyer this summer, locate an $80,000 town house that fits your needs. Although the seller didn't make much of it, you notice from the listing information that it has a $54,000 existing first deed of trust at 9 1/4 percent that is assumable. The loan could be a federally insured FHA or VA mortgage, or one of a large number of existing, conventionally financed loans that are legally assumable with the lender's approval.

A financing package on the town house that would be superior to anything you could get from a conventional lender this summer might work like this: You take over the $54,000 note with principal and interest payments of $494 a month and 21 years yet to run. The seller provides a purchase-money second deed of trust for $20,000 at 11 percent, payable in monthly principal and interest installments of $190 for the first five years. Your out-of-pocket cash up front, exclusive of closing costs, would be $6,000, or 7 1/2 percent down.

Compare that with a conventional bank loan, requiring a minimum of $8,000 down (10 percent), with a fixed annual percentage rate (APR) of 13 percent, including loan origination fees or "points." Monthly payments on that mortgage would be nearly $800 versus the $684 a month through the assumption and seller financing. Moreover, you'd have an extra $2,000 in your wallet to spend on furnishings or landscaping or to put in a savings account.

The downside of the transaction, of course, is that you'd need to roll over the remaining principal balance on the $20,000 note in 1990, which might entail refinancing or securing a new second loan.

But who knows -- maybe you'll be ready to move by (or before) then. Your sale price should more than cover the cut-rate 9 1/4 percent first mortgage and the below-market second. It's entirely possible you'll want to pass along the 9 1/4 percent assumable mortgage to your subsequent purchaser.

After all, fixed-rate assumable loans such as that are bona fide American antiques -- and very much worth preserving for as long as possible. Look for creative ways to use them in a 12 1/2 percent market, and you'll almost always do better than the crowd lining up at the bank window. are bona fide American antiques -- and very much worth preserving for as long as possible. Look for creative ways to use them in a 12 1/2 percent market, and you'll almost always do better than the crowd lining up at the bank window.