The General Services Administration offered to trade away two properties south of the National Mall Monday, including one where 1,500 of its own employees work, in yet another effort to pay for under-funded federal construction projects elsewhere.

Two years ago the GSA floated the possibility of redeveloping five buildings on 22 acres in Southwest D.C. dubbed by the agency Federal Triangle South. The idea, which drew initial interest from 10 developers, that could introduce apartment towers or hotels to an area currently comprised of dated federal offices.

GSA Administrator Dan Tangherlini advanced that idea Monday by offering developers interested in two of the properties the chance to acquire them by providing him construction services he needs on projects that have not received adequate funding from Congress.

Tangherlini proposed trading both the Cotton Annex, an 118,000-square-foot building on 12th Street that has been empty for more than six years, and the GSA’s own regional headquarters, which was built as a warehouse in 1932.

The GSA regional offices at 301 7th Street SW are in a colossal 942,000-square-foot building on three acres, likely worth tens of millions of dollars to real estate firms. Tangherlini said he would make the property available in part by consolidating 1,500 of his staff from that office to the GSA’s main headquarters.

In exchange, Tangherlini is seeking a private sector partner willing to continue work on the GSA’s main headquarters and the long-delayed consolidation of the Department of Homeland Security on the former St. Elizabeths hospital in Southeast D.C.

The GSA completed the first phase of renovations and upgrades to one wing of its main headquarters, at 1800 F Street NW, last year. But GSA officials say they don’t have the money to modernize the 425,000-square-foot western wing.

St. Elizabeths, once a $3.5 billion project to consolidate DHS headquarters units, is now more than $1 billion over budget and a decade behind schedule, according to a House committee report, after Congress repeatedly declined to approve the money President Obama requested to continue it.

A solicitation the GSA posted online seeks companies interested in completing the second phase of the GSA headquarters project and the modernization of up to three historic buildings on the St. Elizabeths campus estimated at about 60,000-70,000 square feet each.

The strategy is akin to the one Tangherlini is taking in pursuit of a new headquarters for the FBI. The government is looking to consolidate the FBI’s headquarter’s staff, currently housed in the J. Edgar Hoover Building and more than 20 leased locations, on a single campus for some 16,000 employees.

Tangherlini described the proposed swap as a chance for the government to maximize the value of vacant or under-utilized real estate. He set a deadline for responses of May 22.

“The Federal Triangle South project is an opportunity to reexamine how the federal government uses these buildings and reassess how this space fits into the surrounding community,” he said in a statement. “This action will facilitate the city’s efforts to transform this precinct that is dominated by federal office buildings, into a mixed-use neighborhood that will both provide for a modern workplace for federal employees and create a vibrant, diverse, and special community of its own.”

Tangherlini is scheduled to testify Tuesday at a House appropriations hearing on President Obama’s budget priorities affecting public buildings and infrastructure.