The project has been starved of funding by both Republicans and Democrats in Congress, causing delays that have in turn inflated the projected price tag as construction costs escalated over time. (Bill O'Leary/The Washington Post)

The Department of Homeland Security is rethinking its plan to build a massive headquarters in Southeast Washington and has proposed a new approach that would cut costs by nearly 20 percent while bringing thousands more employees to the site, according to documents and federal officials.

The headquarters complex — to be built on the grounds of St. Elizabeths Hospital, a former psychiatric facility — is already more than $1.5 billion over budget and 12 years behind schedule.

The DHS is proposing changes that it says would lower the projected overall cost from $4.5 billion to $3.7 billion and speed the timetable for completion by five years, to 2021.

In a briefing Tuesday for the House Homeland Security Committee, DHS officials vowed to save money by reducing the project’s size by nearly 1 million square feet, making better use of existing space and allowing more employees to telework, congressional aides said. The changes would mean 17,000 of the DHS’s 250,000-strong work force — including the homeland security secretary and other top officials — would move to the site. Earlier estimates had been 12,000.

A document prepared by the Obama administration describing the plan called the consolidation of DHS operations urgent because the department is paying for expensive short-term leases around the Washington area.

“The majority of DHS leases in the National Capital Region are expiring over the next five years,’’ it said, adding, “Government-owned space is more cost effective than leasing commercial space.’’

It remains uncertain, however, whether the administration can secure from Congress the roughly $2 billion needed over the next few years to complete the project. Some Republicans have called for the headquarters complex to be dramatically scaled back, and the Government Accountability Office said in September that alternatives should be considered.

Billed as critical for national security and the revitalization of Southeast Washington, the headquarters proposal dates to the George W. Bush administration and was originally scheduled to be finished as early as last year, at a cost of less than $3 billion. The project has been starved of funding by both Republicans and Democrats in Congress, causing delays that have in turn inflated the projected price tag as construction costs escalated over time.

Only a Coast Guard building that opened in 2013 has been completed.

Kamara Jones, a spokeswoman for the General Services Administration, which is developing the St. Elizabeths site for the DHS, said the plan has been revised because of delays in funding and “GSA’s commitment to reducing the federal footprint.’’

“As a result of these factors and the need to use taxpayer money as efficiently as possible, GSA and DHS have been working together for the past year to rethink how many people can be housed at St. Elizabeths and how much space would be required to house them,’’ she said.

Marsha Catron, a DHS spokeswoman, said the revised plan aims to save money by reducing the overall construction at the St. Elizabeths site and the need for space elsewhere.

“Smaller space standards and flexible workplace strategies mean DHS will be able to accommodate additional employees in the space than originally planned. As dependent upon funding, DHS anticipates being able to accommodate additional employees beyond our original estimate,” she said in a statement.

Republicans on the House Homeland Security Committee, who criticized management of the project in a report last year, were cautiously optimistic Thursday.

“DHS’s revised plan to scale down its headquarters construction project at St. Elizabeths is a positive step,’’ said Rep. Michael McCaul (Tex.), who chairs the panel. He added, “As a result of our persistent oversight, tax­payers could save hundreds of millions in reduced construction costs.’’

But McCaul said the DHS must carry out the GAO’s recommendations and “develop contingency plans to adapt to unforeseen events. . . . The committee will continue to review this new plan to ensure taxpayer dollars are safeguarded.”

Rep. Bennie Thompson (Miss.), the committee’s ranking Democrat, said the difficulties facing the project must be viewed in the context of the broader standoff in Congress over the DHS. Funding for the department has been tied up because of Republican objections to President Obama’s executive actions on immigration.

“The St. Elizabeths headquarters project has run into political problems since the start, and the Republican majority, unfortunately, has been hostile to properly funding it,’’ Thompson said. He added, “It’s difficult to be optimistic as Congress has been holding up full-year funding for the department for months.”

In the Senate, Sen. Ron Johnson (R-Wis.), chairman of the Homeland Security and Governmental Affairs Committee, declined to comment through a spokeswoman. Sen. Thomas R. Carper (Del.), the panel’s ranking Democrat, said that funding the headquarters plan is critical but that he has “some questions about the new plan, which proposes to reduce the size of the project.’’

“I am encouraged that the administration is moving forward with the project and has proposed to speed up completion,’’ he said.