Out in Springfield, where "Open House" signs outnumber traffic lights at most intersections, sales agents at The Timbers, a new town house development, had a gimmick and a steady stream of customers yesterday.

The Timbers, like other new subdivisions, has a selling point whose full value the builders couldn't have anticipated when they got mortgage fund commitments from two lenders a year ago: they now offer home buyers mortgage money at 10 3/4 percent interest when most conventional loans these days carry interest charges of from 13 percent to 17 percent.

At a time when some lending institutions have closed their doors to applications for mortgages and when even experts cannot say with certainty whether interest rates are going to go up of down, some home buyers say they are forced to turn to some of the new housing developments that offer lower interest rates.

The Timbers shouted its financing package in four inches of a 10-inch advertisement in the weekend newspaper, and there is no question that it is the drawing card of the moment.

"Everybody that calls, the first thing they ask is do we still have 10 3/4 percent [interest] money available," said Carolyn Bell, saleswoman at the project for the builder, Linpro Co. "It's the biggest selling point a project can have at this pointr. Every time interest rates go up a point, someone is priced out of the market."

Other new home developments report similar response. At the Burke Centre development in Virginia, salesman John Martin said he was swamped with calls from people questioning the advertised 11 percent financing. s

"People think it's a lie," Martin said. "We had one lady who came here and cried because she had her heart set on a house down the street [from her]. When the interest rates went up, it forced her out."

Gary and Regina Reynolds of Annandale were at The Timbers yesterday, back for a second look at the three-bedroom town houses whose prices start just under $80,000.

The Reynoldses, both librarians, had visited the project the previous weekend, but had decided not to place a deposit. Then they watched in horror as some lending institutions raised their interest rates even further last week.

Recalling that two years ago they wanted to buy a house but decided the then 9 percent interest rates were too high, they came back to The Timbers yesterday -- and found that the model that had interested them was sold out.

"Here we are again," Mrs. Reynolds said in quiet frustration. "I almost feel like whatever we do, we'll be wrong." The house they wanted to buy in Annandale two years ago at $65,000 now costs $100,000, Reynolds said, and they do not know if they will qualify to buy at it at any interest rate above 13 percent.

Dan Rosa, who works for the Internal Revenue Service, said he has been searching for a house to buy for two years. He and Margaret Riley, also an IRS employe, were at The Timbers yesterday, looking at the models and talking to sales agents. Together, they have a combined income of about $45,000, he said, and the relatively low interest rate attracted them.

"I can handle a $60,000 mortgage at 10 3/4 percent," said Rosa. "I can't handle it at 14 percent."

Rosa added, "I used to think 8 1/2 was too high . . . . We're finding we have to make sacrifices to by an $80,000 house now."

Saleswoman Bell said that, with a typical $75,000 loan at 10 3/4 percent interest for 30 years, a household would need an income of about $36,000 a year to buy one of the townhouses at The Timbers. That includes payments for principal, interest, taxes, insurance and the homeowners association fee that pays for such things as swimming club membership and removal of snow from the streets.

That same loan, at 15 percent interest, would require an income of $46,500, Bell said.

Builders often get advance mortgage commitments at specified interest rates from lenders well before sales start. For their mortgage money, builders pay a fee that varies. At The Timbers, Bell said, the fee was 1 percent of the total committed amount. If there is a $3 million mortgage commitment, for example, that would be $30,000 -- an amount likely to be reflected in the sales prices of the homes.

Those commitments usually include an expiration date as well.

Real estate agents involved in "resales," or homes that are not new, say they often have to come up with alternative financing arrangements in order to sell homes these days. That could mean persuading the seller to finance the mortgage loan for the buyer, for example.

Business at many firms has been picking up in recent weeks, agents say, largely because March traditionally is the month that begins the best sales period of the year.

Most agents say there is little doubt that soaring interest rates are keeping many people from selling and buying homes now. But they usually advise their clients who can qualify for loans to close their eyes to interest rates, pointing out the tax advantages of homeownership and the fact that home prices will continue to rise and negate a possible drop in interest rates in the future.