Sen. John D. Rockefeller (D-W.Va.) has signed an agreement to buy a seven-bedroom mansion just east of Rock Creek Park in Northwest Washington for $6.5 million, which would be the highest price ever paid here for a residential property.

The large two-story residence, styled much like a southern plantation manor with towering white columns, is at 1940 Shepherd St. NW on 15.9 acres of wooded land in the Crestwood community. The District government currently assesses its value at $1,476,532, making it one of the top 10 assessed residential properties in the city.

The home, long known as The Rocks, was built in 1926, and has been owned since then by the Gaillard family. It is now owned by Mona B. Gaillard, widow of David St. Pierre Gaillard, a one-time investment manager and colonel in the Army Reserves, who died in 1982.

Rockefeller, his wife Sharon and three of their four children have been living in a rented four-bedroom house on Kalorama Circle since early this year, when his term as governor of West Virginia ended and he assumed office as a senator.

Timothy M. Gay, Rockefeller's press secretary, announced yesterday only that Rockefeller had signed "an agreement in principle to buy a Northwest Washington residence valued at $6.5 million near Rock Creek Park" and said the current residents of the house had asked that their names not be made public.

Other sources confirmed that Rockefeller, scion of one of America's wealthiest families, had bought The Rocks.

Gay said settlement on the purchase would occur late this month, but that the Rockefellers would not move into their new home "for some months to come," possibly not until their oldest child, Jamie, 15, returns from boarding school in Massachusetts for summer vacation.

Over the last several months, the Rockefellers searched the District for a suitable residence, at one point looking at the Brady Estate at 1801 Foxhall Rd. NW, which is currently the highest assessed residential property in Washington at$5,255,157 for the tax year starting July 1.

Most of the city's one-time grand estates have been subdivided for construction of luxury town houses, such as the one once owned by Rockefeller's late uncle, former vice president Nelson A. Rockefeller.