The Alexandria City Council, in an aggressive campaign to preserve the Old Town area, is urging residents in the city's crowded Historic District to relinquish permanently their right to build on the open spaces in their private lots.

The council unanimously agreed last week to create an open-space easement program "for civic-minded property owners to foster the protection and preservation of open space." The easements are permanent, and subsequent owners would be bound by their terms.

Virginia has a program whereby residents with two or more acres can gain tax benefits by agreeing not to develop their property. The Alexandria program would encourage those with even small gardens or yards to turn over their development rights.

Vice Mayor Patricia S. Ticer, who introduced the proposal last year and has headed a study group on the issue since then, said participants might receive a small plaque and a tax break for signing the easements, but mainly they would gain civic pride by "contributing to the good of the city."

"There is no pressure that you can force on people to make them sign easements," Ticer said. "They have to want to do it."

Council member Caryle C. Ring Jr. said the program has a great deal of support in the community and would include only those who volunteer. The city's study group identified more than 100 potentially eligible properties in the Historic District.

Although some homeowners said yesterday that they would not sign easements themselves, they agreed that others should be able to do so. A developer who has worked on several projects in Old Town said he opposes the easements. "It's my opinion that it reduces the value of the ground for future development," he said.

John Howard Joynt, an Alexandria resident who bought his 18th century house at 601 Duke St. in 1936 and signed an easement on the property last year, said: "We love this house; we appreciate it; we want it kept for posterity." Joynt urged others with historic property to guard against further development in Old Town by signing easements. "Who wants this overcrowding?" he asked.

Although the Alexandria program will allow those with historic properties like Joynt's to sign easements, Ticer said it is aimed primarily at those "in the areas that are not necessarily so historic or scenic."

The program will concentrate initially on Alexandria's Old Town area, Ticer said, but, if successful, could spread to all areas of the city. "People in the last few years have been building on every inch of property," Ticer said. "Everybody feels the suffocation level rising. Light and air enhance your quality of living; the walling-in of a city reduces its livability."