The Arlington County Board approved the construction of 10 townhouses behind the historic Glebe House yesterday under a plan that will give neighbors who oppose the project 90 days to find an alternative to it.

The board voted 4 to 1, with member John G. Milliken dissenting, to allow Preston C. Caruthers, a prominent local developer, to build the 10 "clustered" townhouses on the two-acre lot at 4527 N. 17th St., the site of the Glebe House, which will be preserved under the plan. A portion of the house dates to 1770 when it was built as a minister's rectory for the Fairfax Parish during the time that George Washington was a parish vestryman.

In a related vote after a four-hour hearing on the proposal at which 37 persons spoke, almost evenly divided on the issue, the board decided unanimously to approve a use permit for the National Genealogical Society to occupy the house.

The society, which wants to locate its national headquarters in the house, plans to lease it for two years and then buy it.

"The plan would be preferable to another, possibly more profitable plan of tearing down the Glebe and filling the whole area with new dwellings," said James E. Huddleston, former president of the Arlington Historical Society, which endorsed the plan.

Yesterday's board action had been deferred a month at the request of the Waverly Hills Civic Association, whose members opposed the townhouse plan and wanted time to negotiate with Caruthers and property owner Frank Ball Jr. on other options for the property.

David K. Martin, the association's president, said yesterday that the neighbors hope to raise funds themselves to preserve the property as it is or find another developer with a plan more compatible with the single-family homes in the area.

"We're going to give it our best shot," Martin said.

Ball, who has been trying to sell the property for more than three years, said yesterday that Caruthers is the only builder to present a viable plan that would save Glebe House. Other developers, he said, may want to raze the historic house.

Caruthers, who could have built 14 houses under the existing zoning, agreed to the proposal on the condition that any new developer who won approval reimburse him any costs he has spent on the project, including the $475,000 purchase price and an estimated $50,000 in renovations to Glebe House.

All five board members said they wanted to preserve Glebe House and, with the exception of Milliken, said they believed Caruthers' plan was the best solution available. Milliken said he objected to the density and bulk of the townhouses, contending they will be out of character with the neighborhood.

In other action, the board deferred a decision on a request from MCI Communications Corp. to erect three lighted rooftop signs on its two Pentagon City buildings. The National Capital Planning Commission and the Army Corps of Engineers have complained that the signs would be visible from Arlington Cemetery, the Mall and the George Washington Parkway.