Looking for 13 percent mortgage-interest rates in a 16 percent market, or a purchaser for your home despite record financing costs?

Then consider the newest wrinkle in the national housing finance field: custom-tailored "buy-down" plans by sellers, parents or friends on behalf of home buyers.

In plain English, buy-downs are short-term subsidies of mortgage interest rates, designed to make homes more affordable to purchasers during the first new years of ownership.

Builders use them frequently; the 11 percent to 13 percent financing offers you see in the weekend real-estate advertisements are virtually all buy-downs.

The homebuilder pays a lender a lump sum to subsidize the purchaser's loan rates for the early years of a mortgage. If the monthly payment differential between a $60,000 loan at 16 percent and the same loan at 13 percent is $140, for instance, the builder agrees to contribute one or more years' worth of monthly subsidies on the buyer's behalf.

The builder may tack all or part of that subsidy onto the selling price of the house, or may not, depending upon market conditions.

Once the buy-down period ends, the homeowner's interest rate jumps back to the original rate. If rates have fallen in the interim, the homeowner can refinance the loan.

Buy-downs with lending institutions have been one of the most important techniques sustaining the new-home market during the past year, but have been rarely used in resale transactions.

That's about the change, however, thanks to a new program for lenders being launched by Fannie Mae -- the Federal National Mortgage Association.

Fannie Mae, the $60 billion, privately run behomth of the housing-finance field, will now purchase buy-down loans from mortgage bankers, savings and loans, commercial banks and other lenders.

More importantly, Fannie will purchase custom-crafted, home seller-subsidized loans made and serviced by these lending institutions. The buy-down features of the mortgages can take a wide variety of forms -- ranging from nonrepayable direct subsidies (like the builders' buy-downs) to repayable second mortgages.

The repayable, second-mortgage buy-down is probably the most innovative approach and deserves a close look by home buyers as well as sellers. It's not anywhere near as complex as it sounds.

Let's say, for example, that you and your spouse are young, first-time home buyers. You've studied the market, and located a $65,000 townhouse or condominium that will meet your needs for the next couple of years. But you've struck out on financing. The seller says his mortgage isn't assumable and he can't assist you in any "creative" way with your own financing. Local lenders say your combined salaries qualify you for an 80 percent, 12 3/4 percent loan on the house, but not the 15 3/4 percent they need to charge you today.

What do you do? Consider proposing a repayable buy-down on your mortgage rate -- by the seller or by a parent. If the home seller is to be the subsidizer, the arrangement could work like this: You'd agree to a sale price slightly above what you'd otherwise offer -- say, $66, 0.

The seller would agree to subsidize your montly payments for up to three years to bring your interest rate down to 12 3/4 percent. Out of the $690 owed each month on your $53,000 mortgage at 15 3/4 percent, your seller would agree in advance to pay $125 -- cutting your out-of-pocket cost to $565.

The maximum subsidy to you by the seller during the three years would be $4,500 ($125 times 36 months). Repayment of the subsidy amount would be guaranteed to the seller by using the house as collateral, via a second mortgage at some moderate rate, such as 12 percent.

Under the buy-down agreement, however, the seller woudl collect no interest or principal during the three-year buy-down period. In effect his loan to you would be what is known as a "sleepy second." It would be paid off, with interest, only when you sold or refinanced the property. In this case, you'd agree to refinance -- hopefully at a rate lower than 15 3/4 percent -- after three or four years.