The Alexandria City Council, under pressure from residents of historic Old Town, approved an ordinance last night limiting the height of new buildings in the neighborhood to no more than 30 feet without special council permission.

By a 5-to-1 vote, the council reduced the permissible height of new construction by 20 feet. Old Town residents had sought the change for a year, saying that potential condominium developments could dwarf the two- and three-story row houses that are common in the neighborhood.

Under the new ordinance, developers may erect no building higher than 30 feet unless they obtain a special use permit from the council. The maximum height with such a permit would be 50 feet. The previous law allowed heights of 50 feet without council action and 70 feet with a special use permit.

The ordinance took on particular urgency for Old Town residents several weeks ago, when Alexandria developer Lawrence Brandt proposed a four-story, 100-unit condominium on South Union Street along the Old Town Potomac River waterfront. The Old Town Civic Association, a group long active in local politics, had sought the change in part to help block Brandt's building.

"We believe that lower heights may be appropriate in the waterfront area," said Robert Dempsey, the association president.

The ordinance was opposed by several developers and owners of waterfront property that is used for commercial purposes, who contended that it was too restrictive. The only council member who voted against the ordinance, Robert Calhoun, said he opposed it because developers would almost certainly sue the city in an attempt to overturn it.

"In passing this ordinance, you are inviting the danger of a legal attack, one in which there is an arguable risk of not prevailing," Calhoun told council members. "The outcome of losing could have severe repercussions {by setting a bad legal precedent for other zoning cases}."

But several council members who had initially shared Calhoun's concern said they voted for the ordinance to show residents that they were serious about controlling growth in the city. "I'm reconsidering {my opposition to the ordinance} because the perception of control is not there," said Vice Mayor Patricia S. Ticer, who voted for it. Council member Carlyle C. Ring was absent from the meeting.