Fairfax County zoning laws are causing problems for two landowners along the banks of the Potomac River in one of McLean's most affluent neighborhoods.

Diane Kay, wife of Washington area developer Allan Kay, and Elaine D. Rosensweig own separate parcels of land along the river off Chain Bridge Road near the George Washington Parkway.

The Kay family is renovating the Merrywood estate, the childhood home of Jacqueline Kennedy Onassis. It wants to build a new bath house and other so-called outbuildings on the seven-acre site. Rosensweig wants to build a single-family home on 5.4 acres of nearby waterfront property.

The land is zoned R-20 for high-density, multifamily housing at 20 units per acre, however, and only luxury homes and town houses exist in the area.

County zoning regulations will not allow for construction of additional single-family detached houses, or bath houses, guest houses or other buildings that might be considered accessories to single-family homes.

Diane Kay and Rosensweig have hired lawyers and asked Fairfax County to change the zoning from 20 units to the acre to an R-E, residential estate category.

Such a change would constitute downzoning, reducing the density allowed for construction on a particular site. Fairfax officials have shied away from downzoning since lawsuits were filed challenging the right of the county's board of supervisors to downzone. The country prevailed in those suits, which were filed after a sweeping downzoning of land in the Occoquan watershed.

Early this summer, supervisors refused to change the county's comprehensive land-use plan for a small home site owned by a family living on a fixed income near Washington Dulles International Airport even though the board had changed the land-use plan for the house from residential to industrial a few years ago. This year, owners asked that the house be returned to residential status because they feared increasing industrial tax rates.

One supervisor said the decision was made not to change the plan because it would amount to downzoning and board members feared another Occoquan lawsuit.

Zoning attorney William Hansbarger, representing Kay, said the change simply would bring the Kay home site into conformance with existing homes in the area. Supervisor Nancy Falck said the same thing when she asked supervisors to grant expedited hearings and allow the Department of Environmental Management to accept preliminary building applications for the two sites. Zoning officials have told Kay and Rosensweig they can submit applications but the decision to accept those applications do "not imply a favorable staff recommendation" or eventual board approval of the requests.

Attorney Barnes Lawson, representing Rosensweig, told county officials that county records indicate single-family homes were allowed "at the time the property was bought.". However, an update of the county land-use plan dubbed the area for R-20 development, Hansberger said.

"We have the unique situation where the applicant must rezone its property from an intense high-rise-apartment category to a limited, single-family-detached category in order to construct such a dwelling," Lawson told county officials.

Public hearing dates have not been set by the planning commision or the board of supervisors.