The Fairfax County Planning Commission is recommending approval of zoning changes that would allow owners of four of the most valuable properties along the Potomac River in McLean to expand or build homes on land now zoned for high-rise apartments.

With little fanfare, commissioners decided to back the downzoning of 18 acres of land, including the Merrywood estate, the childhood home of Jacqueline Kennedy Onassis.

The Fairfax Board of Supervisors is scheduled to act Monday on the proposals, which are part of a tangled mess in Fairfax County's zoning laws, county staff members said.

If the Fairfax board approves the change, Diane Kay, wife of developer Alan Kay, will be allowed to build a new bathhouse at the swimming pool at the Merrywood estate, which she purchased several months ago from former NBC anchorwoman Nancy Dickerson. Kay discovered she could not rebuild a bathhouse that had been torn down without getting the zoning changed, according to county officials, and is seeking a change from R-20, one of the county's highest-density residential classifications, to residential estate.

Other applicants seeking changes to the residential estate category are T. Eugene and Joan Smith, former secretary of Labor Raymond Donovan, and Samurai Venture, which is owned by Elaine D. Rosensweig, a resident of Kalorama Road in the District.

The proposals were grouped together and slated for quick action at the request of Dranesville Supervisor Nancy Falck and her fellow members of the board of supervisors. The county faces a backlog of pending applications for land-use changes.

The sites involved are between the existing Merrywood on the Potomac town-house development and the Potomac River, close to the estates of Gov. Charles Robb, developer James T. Lewis and Sen. Edward M. Kennedy (D-Mass.). None of the sites has road frontage, but the planning commission is recommending that traditional road-frontage requirements be waived.

In a controversial rezoning in 1962, Fairfax rezoned the property involved for high-rise apartments. The zoning category on the books at that time allowed single-family homes to be built on such sites. However, changes enacted in 1978 prohibited single-family houses in the new category, setting the stage for the current attempts to change the land-use status.

After the land was set aside for high-rise use, "a scenic easement was placed on the property" by the Department of the Interior, prohibiting high-rise development, according to the county staff. The site later was approved for construction of the existing Merrywood luxury town-house development.

The rezoning requests, if approved Monday, would amount to what is known as "downzoning." Although the county has shied away from downzoning any land since its controversial downzoning of enormous acreage in the Occoquan watershed, some county officials said the situations are not comparable.

The changes, according to Falck's office and planning staff members, will bring the sites into "conformance with existing uses of nearby parcels." Several of the sites already have houses on them.

Two of the landowners involved have promised not to seek a future subdivision of the land they own, an action that ensures the properties will remain estates, county officials said.