It sounded like a great marketing idea, officials of Gulf Reston, the developer of the new town of Reston, agreed.

Let's decorate a model house with $30,000 worth of furnishings from Bloomingdale's, they decided, call it the "Bloomingdale's house" and reap a promotional bonanza.

But the bonanza almost became a bust when residents of Wethersfield Cluster, where the Bloomingdale's House was put together, said they didn't like the idea and threatened to seek a court injunction stopping the house from being opened.

After some last-minute negotiations, during which Gulf Reston made some concessions to protect the privacy of cluster residents, the Bloomingdale's house was opened last weekend. It was visited by more than 6,000 people - without incident, a Gulf Reston official said.

The developer agreed to hire a security guard to keep visitors from parking in the resident's lot, to pay for a nighttime security patrols to discourage burglas who might be attracted to the cluster by the $30,000 furnishings in the Bloomingdale's house, and quit using one model house as a sales office for other new clusters. Gulf Reston also agreed to give residents dashboard signs that got their cars past the security stationed at the entrance.

"Both parties are happy with the agreement," cluster president John Rogalski said. "It's fair and reasonable to the residents. It brings Gulf Reston in substantial compliance with the convenants and the zoning ordinance."

"I was shocked by the reaction," Lee Shur, vice president for marketing at Gulf Reston, said. "We thought the people would be proud we were using their cluster as a showcase."

But, as Rogalski said, Wethersfield's residents were annoyed that they hadn't been consulted about plans for the promotional house. They made the discovery through hearsay.

Shur explained: "We didn't call them earlier because it's confusing and difficult to work out a program when everybody is involved from the beginning."

Gulf Reston wound up holding three meetings with the cluster's directors before an agreement was reached. The chief feather smoother was the general manager of the firm, Francis C. Steinbauer.

Though Bloomingdale's is a joint beneficiary of the promotion, the New York based department store chain, which has new stores at Tysons Corner and White Flint, didn't have to spend a cent.

The entire tab was picked up by Gulf Reston, which paid the retail price of the furnishings, and even had to lay out a fee to borrow the promotional expertise of Bloomingdale's Washington public relations director, Teddy Westreich.

The chief purpose of the Bloomingdale's house is to sell houses, not sofas, draperies, glassware and other furnishings from the department store, according to Gulf Reston's public relations director, Peter L. McCandless.

"Many people think Reston is completed," he said. "We felt we needed to do something dramatic to tell people we're still growing."

Marketing man Shur put it a little more directly: "The hook is the Bloomingdale's name. That gets people here. Then we can explain to them about the houses."

Of the 61 townhouses built in Wethersfield, located in the developing South Lakes section of Hunters Wood, Gulf Reston has 21 left to sell.