A Washington developer who plans to build 120 luxury homes on the Foxhall Road estate of former Vice President Nelson A. Rockefeller has agreed to neighbors' demands that he sharply alter the layout of the development and leave about 30 percent of the land undisturbed.

The novel agreement between the Rozansky and Kay Construction Co. and a group called the Coalition for Planned Environmental Development also permits the citizens to monitor construction of the $300,000 to $400,000 homes and to have an outsider arbitrator decide any unresolved disputes.

Rockefeller's neighbors - some of the wealthiest and most prominent in Washington - felt that a preliminary Rozansky and Kay plan last year calling for scattered construction of up to 131 homes on the 25-acre estate would result in a "ticky tacky tract development" much like that seen in Gaithersburg and Dale City.

But under the agreement, Rozansky and Kay says it will cluster the homes in groups of six to 12, while leaving a 30-foot buffer of trees around the entire estate, at 2500 Foxhall Rd. NW. In addition, one other section in the center of the land will be left undisturbed, according to lawyers for both the developer and the coalition.

In all, about 50 percent of the land will either be left untouched or relatively, so, according to Charles Donnenfeld, the developers attorney.

Donnefeld and Tommy H. Jackson, chairman of the coaltion, also said the developer has agreed to plant more trees on the property, some of them by two, three and four inches in diameter.

"We had a big discussion on how big the replacement trees whould be," Donnefeld said. "It could run into hundreds of new tree." Jackson said the pact has "specific provisions with regard to the percentage of trees that may be cleared (50 percent) and over that percentage they'd have to replace them."

The new development plan was drawn by architect Arthur Cotton Moore and an associate, Barry Dunn, who were hired by Rozansky and Kay. But the coalition also hired an architect, Richard Ridley, who advised the groups, as the plan was drafted.

Alan Kay, owner of the develpment firm, said the 30-room Rockefeller house and said 1 1/2 acres surrounding it are for sale for $800,000. But he said that if it is not sold by the time the firm is ready to build homes on the adjoining land - probably by january - the house will be demolished.

Kay said construction on homes will start by the end of the summer, but that 70 people are already on a list waiting to buy them. He said 15 prospective buyers have sent the firm deposits of from $5,000 to $25,000, but that the money was returned pending completion of designs of the homes.

He said the homes will range in size from 3,200 to 4,000 square feet and will have such features as plaster walls and elevators.

Jackson said that while leaders of the coalition would still prefer that the $5.5 million estate be sold to a single owner and kept in its present state, they are pleased with the changes the developer has agreed to make.

"It was not our idea to put up our hands and say Stop,'" Jackson said. "We talked with several institutions, schools and conference centers (about buying the property). There was a lot of interest, but the price was one that drove everyone out of the market.

"We said we would oppose unreasonable development," said jackson, an attorney who lives five blocks north of the estate. "We now have what we believe is reasonable development."

Both Donnenfeld and Jackson said the agreement was particularly unusual because of the arbitration clause in the event of future disputes and the fact that a negotiated settlement was worked out before city permits for construction were sought. In addition, the negotiations were unusual because the property is already zoned for construction of houses on every 7,500 square feet, just as the developer originally proposed, although not in a cluster format.

"Because of the concern that has been expressed by the community we went through this elaborate prcess," Donnefeld said. "I hope it works."

However, one other source close to the negotiations said the developer engaged in the discussions and made the changes to preclude either side from taking the dispute to court.

The agreement, which has been approved unanimously by the coaltion's steering committee but still must be voted on Thursday by members of the coaltion, prohibits the group from suing the develper over the project.

"I think it's a good agreement," Jackson said, while conceding he is not sutre how the coalition's members will react to it.