The Loudoun County Board of Supervisors agreed last week to allow the town of Leesburg to annex a 7.l square mile tract, and Leesburg in turn waived its right to become a city until 2009.

In an agreement negotiated between the town and county over the past seven months, Leesburg will take in land to the north, south and east of its current boundaries. The agreement, which settles a lawsuit, must be completed by court proceedings. The annexation, scheduled to take effect Jan. 1, 1984, will add 1,400 new residents to the town of 8,350.

"This agreement will be a great benefit to Leesburg," said Town Manager John Niccolls. "The major rubbing point between the town and county has been that of Leesburg wanting to become a city sometime in the future, and that point has been resolved."

Under the agreement, Leesburg regains its full rights in the year 2009, by which time, Niccolls said, population in the town might make it feasible to become a city. By remaining a town, Leesburg residents continue to pay county taxes and to receive county services. Residents also pay taxes for extra amenities in Leesburg.

The agreement is contingent on legislation pending in the Virginia General Assembly that would give towns specific authority to waive their rights to become cities.