New York-based Monday Properties plans to begin work soon on what it says will be the Washington area's tallest office building, a 30-story Rosslyn tower twice the height of the tallest office buildings in the District.

The building, planned for 1812 N. Moore St. and approved by the Arlington County Board two years ago, required special permission from the Federal Aviation Administration because of concerns about air traffic from Reagan National Airport. Rising 390 feet, it will feature stunning views of the Mall, have a shimmering glass exterior and be one of the most energy-efficient buildings in Virginia. Construction is scheduled to start Oct. 14 and continue until early 2013.

Other than on Pennsylvania Avenue NW, Congress banned buildings of more than 130 feet in the District. But Rosslyn is not bound by the height limit. Rosslyn already boasts two of the region's tallest buildings, at 1000 and 1100 Wilson Blvd., also owned by Monday. Another developer, the JBG Cos., is planning a tower of nearly identical height down the block on North Moore Street that will feature an observation deck.

The two developers have a size-driven rivalry, with Monday issuing a news release calling 1812 North Moore "D.C.'s tallest building" and JBG marketing its project, Central Place, as "towering high above the Rosslyn Metro station - and the competition."

Anthony Westreich, Monday Properties president and chief executive, said that companies in downtown Washington have realized that they can find less-expensive space with better views just across the river from Georgetown. As a result, he said, "you'll see many more D.C. tenants coming to the Rosslyn-Ballston corridor."

Because of lower operating expenses and real estate costs, Westreich said a company renting 200,000 square feet (about one-third of what Monday is planning) could save as much as $100 million over the course of its lease.

"You look at the economics of a 10- or 15-year lease, the savings are enormous," he said.

But perhaps most striking is Monday's decision to build now. The developer has no tenants signed for the building and, rather than trying to find banks or other lenders to share the risk of the $300 million project, is paying the first $30 million. Building speculative office buildings is what got many developers in trouble during the most aggressive years of the real estate boom.

But Westreich said the tenants, and the investors, would come once the project started. "We're in deep negotiations with five different tenants right now," he said.