The Foxhall Road estate of former Vice President Nelson A. Rockefeller has tentatively been sold for $5.5 million to a Washington development company that has plans to build 100 new houses on the 25 acres in one of Washington's poshest neighborhoods.

While Rockefeller's office announced yesterday that the sale is "conditional" and not yet "a matter of public record." real estate and other sources here indicated the deal is very close to completion with Rozansky and Kay Construction Co., a longtime area developer.

The rolling, heavily treed terrain where the firm plans to build the houses was purchased by Rockefeller in the mid-1940s. He frequently lived there when he was Vice President. Sources said the new houses will begin in six months if all goes smoothly, the sources said.

If the deal goes through and the development takes place, it could alter the character of one of the loveliest areas in the city by increasing the density of housing there.

On the other hand, a city planning and zoning official, Kirk White, said that such a development "could be very pleasant if it were done nicely... There are very (steep) slopes and trees that should be preserved."

White said that while the city "wants to see these things protected," it does not have authority to oversee general development plans for the area. He said the city may be able to exercise some control over what happens to the terrain itself through enforcement of environmental protection requirements.

While some neighbors expressed dismay at hearing the news from a reporter yesterday, there was no general uproar and most declined to t be quoted.

The Rockefeller estate is zoned R-1-A, which allows a house to be built on every one-fifth acre. White said there would be no zoning impediment to a development of 100 new houses on the site. He said that no effort to change the zoning to prevent such a development could be mounted since the zoning is "already the most restrictive in the city."

"It's the zoning pattern of Spring valley," White said, referring to the area southwest of Massachusetts Avenue, a pleasant and expensive neighborhood similar to much of the Foxhall Road area but lacking the occasional large estates located in the later.

Foxhall Road, despite its problems of traffic congestion, remains one of the best addresses in the city. Washington arts patron David Lloyd Kreeger, the founder of Government Employees Insurance Company (GEICO), and Laughlin Phillips. publisher of the Washingtonian Magazine, are among those who reside in the area. The German ambassador's residence is located down the road from the Rockefeller estate. and the Belgian ambassador's residence, considered one of the nicest in the city, is also located in the area.

It was not clear yesterday, what will happen to the original white frame farmhouse, which was built about 90 years ago on the site and which was expanded by Rockefeller to seven bedrooms. There is a swimming pool and a tennis court on the property, as well as a pond.

The reported asking price when the property went up for sale early this year was $8 million, and Rockefeller was said to want cash.

Details of the proposed financing of the deal, and of what exactly is holding up the settlement, were not available yesterday. Officials of Rosansky and Kay refused to comment and Rockefeller's office in New York issued a terse statement saying that the sale agreement is "subject to certain conditions which have not yet been waived or satisfied."

The estate was assessed by the city for tax purposes at $2.1 million for fiscal 1976, the second highest residential property assessment in Washington. The highest was the $2.9 million assessment of the John Archbold property on Reservoir Road NW.

A Rockefeller spokesman said of the house's contents last year, "It's not one of the gallery houses," referring to the art collections in other Rockefeller houses.

The Rozansky and Kay Development Co. is run by Allen Rozansky and Alan Kay, local men who went into real estate sales and development and did well throughout the Washington area, especially during the building boom times of the 1960s.

Alan Kay was one of the developers of the controversial Greenbriar apartment in Greenbelt, Md. The development was cited in court for a building code violation and was for a time in the early 1970s the center of disputes over its sewage treatment facilities.

Neighbors reacted to the development plans in a low-key fashion.

"We were sort of hoping for maybe 10 or 12 nice large houses, with yards," said Col. Stanley Lewis, the head of the North Foxhall Road Citizens Association.

"I was sort of hoping an embassy would buy it, " said his wife.

Ruth Rhetts, whose house abuts the Rockefeller woods, said she had sold a small portion of her land to the developers, who evidently wanted it for road access. She said several other neighbors had been approached to make similar sales.

"It will be sad for me to see this go." Mrs. Rhetts said, gazing out her window at the Rockefeller trees just beginning to show gold. "But you can't keep it forever."