Federal Triangle is a triangular area in Washington, D.C., formed by 15th Street NW, Constitution Avenue NW, Pennsylvania Avenue NW, and E Street NW. Federal Triangle is occupied by 10 large city and federal office buildings, all of which are part of the Pennsylvania Avenue National Historic Site. Seven of the buildings in Federal Triangle were built by the U.S. federal government in the early and mid-1930s as part of a coordinated construction plan that has been called "one of the greatest building projects ever undertaken" and all seven buildings are now designated as architecturally historic.

Federal Triangle station is the Washington Metro station serving Federal Triangle and its immediately surrounding areas.

The name "Federal Triangle" appears to have been a journalistic invention. The press wrote of a "Pennsylvania Avenue Triangle" as early as November 18, 1926, and use of this name continued as late as June 1929, but by 1927, it was more common for the news media to refer to the area as "the Triangle". This name was in use by 1928, even by government officials, and still used as of late 1934.

The Associated Press first used the term "Federal Triangle" (with both words capitalized) in 1935. The New York Times first used the term in 1936, although the paper's capitalization of both words did not become standardized until 1939. Definitions of the area also varied at first. Early news reports believed the eastern apex of Federal Triangle extended as far east and south as the Ulysses S. Grant Memorial in front of the United States Capitol. But almost all reports referred to Pennsylvania Avenue NW and 15th Street NW as the Triangle's northern and western boundaries.

The Senate Park Commission, also known as the McMillan Commission, was formed by the United States Congress in 1900 to reconcile competing visions for the development of Washington, D.C., and especially the National Mall and nearby areas. The commission's plan for development, the McMillan Plan, proposed to replace the residences and other buildings on Lafayette Square with tall, Neoclassical office buildings with facades of white marble for executive branch offices. Demolition proceeded on some nearby buildings: notably, the Hay-Adams Houses, Corcoran House, and a portion of the Decatur House grounds. But the rapid expansion in the size and number of executive-branch agencies in the 1910s, 1920s, and 1930s made the McMillan Plan's development of Lafayette Square impractical, and Congressional and local support for the project waned.

Over the next few years, the president and Congress established several agencies to supervise the approval, design, and construction of new buildings in the District of Columbia: The Commission of Fine Arts in 1910 to approve designs, the Public Buildings Commission in 1916 to make recommendations about the housing of federal agencies and offices, and the National Capital Parks and Planning Commission in 1924 to oversee planning. In the mid-1910s, Congress appropriated and the government spent $7 million to acquire land on Pennsylvania Avenue NW between 14th and 15th Streets NW and several blocks south. No demolition or construction took place during that period, and the government merely collected rent from the tenants in the area. In 1924, the Public Buildings Commission recommended that a new series of federal office buildings be built near the White House. The plan called for a complex of buildings to be built at Murder Bay, a muddy, flood-prone, malaria-ridden, poverty-stricken region lacking in paved roads, sewer system, and running water and almost exclusively home to numerous brothels and an extensive criminal underclass.

Federal Triangle, as the area would be renamed, had its genesis in 1926. An attempt to provide $50 million to fund, among other things, a national archives building and develop federal offices along Pennsylvania Avenue NW was proposed in 1925. The effort saw success in 1926 with the passage by the United States Congress of the Public Buildings Act, which authorized the construction not only of the Federal Triangle complex of buildings but also a new U.S. Supreme Court building opposite the United States Capitol, a major extension of the U.S. Government Printing Office building on North Capitol Street, and significant widening of B Street NW on the north side of the National Mall (eventually renamed Constitution Avenue). However, appropriations were to be made annually, leaving control of the project firmly in Congressional hands. Congress appropriated $50 million, including $10 million annually for five years, for construction of these projects in 1927, with half the funds to be spent solely on Federal Triangle. A second appropriation bill provided $25 million for buying up all additional privately held land in Federal Triangle. On June 5, 1926, the Treasury Department, which had been given authority over the implementation of the building program, announced the Federal Triangle projects (among others) which would move forward and their anticipated cost:

• A National Archives building, with total cost of land and construction to be $6.9 million, including $1 million appropriated in fiscal 1927.
• A new Internal Revenue Bureau building, with total cost of land and construction to be $7.95 million, including $1.7 million appropriated in fiscal 1927.
• A new Department of Commerce building, with total cost of construction to be $10 million, including $600,000 appropriated in fiscal 1927.

Treasury officials said the Archives building was their top priority, followed by the Internal Revenue building, two Department of Agriculture projects, and the Commerce building last. At that time, no provision was made to construct a new building for the Department of Justice and no sites were named for construction of the three announced buildings. Preliminary plans for the Commerce building were presented to the Commission on Fine Arts and Public Building Commission in mid-June. On July 7, the Treasury Department and Commission of Fine Arts announced sites and sizes for the three previously announced structures. The Department of Commerce building would contain 1 million square feet (93,000 square metres) of office space and be sited on the south side of B Street NW (now Constitution Avenue NW) on the National Mall. The Internal Revenue building would contain 650,000 square feet (60,450 square metres) of office space and take up two whole city blocks between 10th and 12th Streets NW and B and C Streets NW (cutting off 11th Street NW). The National Archives would contain 2.3 million square feet (213,900 square metres) of office space, and take up one city block between 12th and 13th Streets NW and B and C Streets NW (cutting off the last block of Ohio Avenue NW). The government owned three of the four plots needed for the Internal Revenue site, but none of the land beneath the proposed Archives building. Purchasing both sites, officials estimated, would cost $700,000 each. Treasury officials also proposed at this time adding a Justice building on Pennsylvania Avenue between 14th and 15th Streets NW, and a Labor building (facing 15th Street) between 14th and 15th Streets NW and D Street NW and Ohio Avenue NW. Preliminary plans for these buildings were expected to be presented in three months.

The purchase of land delayed the construction program considerably over the next several years. Center Market, designed by architect Adolf Cluss and built in 1872, was the largest of the District of Columbia's markets, serving tens of thousands of people a day at a time when general stores and greengrocers were uncommon in the city. It was also a hub for transportation in the District of Columbia, as the city's trolley lines converged there. At the time it was built, it was the largest food market in the United States—with space for more than a thousand vendors, the city's first cold-storage vaults, its own ice storage facility, and its own artesian well. Center Market, however, occupied two blocks between 7th and 9th Streets NW on the north side of B Street NW. As early as August 1926, planners recognized that relocating Center Market and purchasing land from owners eagerly seeking inflated prices from the federal government would delay the Federal Triangle project significantly. Early negotiations with private landowners in the area collapsed early on when owners demanded exorbitant prices for their properties, and the city and federal government began condemnation proceedings in late August 1926 against owners on B Street NW between 10th and 13th Streets. Federal legislation authorizing expanded, faster condemnation powers for the Federal Triangle areas was sought in November 1926, and passed a month later. Condemnation (under the old eminent domain law) of the final block necessary for the Internal Revenue building began in January 1927. The Commission of Fine Arts placed a ban on all non-federal construction in the area in February 1927. The relocation of Center Market began in July 1927. The final lot for the Internal Revenue site was not condemned and purchased until October 1927. Negotiations for the privately owned land at the Archives site began in late November 1927. Funds were furnished in February 1928 to buy the Southern Railway building at the southwest corner of 13th Street NW and Pennsvylania Avenue NW, which already housed a number of federal agencies (it was purchased in 1929). After six months, D.C. city officials finally began to consider a new location for Center Market. The new 1926 federal condemnation law was first used in October 1929 to condemn a set of parcels on the south side of D Street NW between 13th and 13½ Streets NW. A second set of parcels (Pennsylvania Avenue NW and B, 12th, and 13th Streets NW) was condemned under the new law in December 1930. The first land for the National Archives (later the Justice Department) building site was not acquired until July 1930 even though the site had been selected for development in November 1926. Center Market was not relocated until early 1931, more than four years after the process began. Additional land for the Justice and Post Office buildings was condemned in March and December 1931. Another major effort had to be made to condemn and remove railroad tracks from Federal Triangle, which had converged on the Center Market site. Although the Treasury Department had ordered the tracks lifted by April 1, 1931, this effort did not begin in earnest until early 1931. Negotiations over the price of the land and equipment broke down in February 1931, and the tracks had still not been removed by January 1932. Delay occurred in obtaining the Post Office land as well. Several parcels of land were not condemned until July 1, 1931—a single day before demolition on adjacent parcels of land began. The land for the Apex Building site was finally obtained through condemnation in July 1931.

The initial Federal Triangle building plan was significantly revised by the Public Buildings Commission in November 1926. President Calvin Coolidge refused in September to permit the Commerce building to be placed on the Mall. A few weeks later, the Commission of Fine Arts decided that the Commerce building should be relocated to 14th and 15th Streets NW, extending from D Street NW to B Street NW (cutting off Ohio Avenue NW and C Street NW). The National Capital Parks and Planning Commission established a committee (composed of William Adams Delano, Milton Bennett Medary, and Frederick Law Olmsted Jr.) to study the street plan in the Federal Triangle area and recommend appropriate closures or alterations (if any). While the Public Buildings Commission studied the Commerce site (and even considered halving the size of the building so that two structures could be built along 15th Street), plans for the Archives building were approved and a contract signed for razing of the Internal Revenue site. After these deliberations, the Public Buildings Commission announced on November 17, 1926, that several new buildings would be added and new sites for proposed buildings announced, including:

• A new Department of Justice building, to be located between Pennsylvania Avenue NW and D Street NW, and 14th and 15th Streets NW.
• A new "General Supply" building, to be located between 14th and 13th Streets NW between D and C Streets NW.
• A new Independent Offices building, to be located between 12th and 13th Streets NW and B and C Streets NW (cutting off the last block of Ohio Avenue NW; this was the original proposed site of the National Archives in June 1926).
• A new Department of Labor building, to be located between 13th and 14th Streets NW and B and C Streets NW.
• A new General Accounting Office building, to be located between 9th and 10th Streets NW and B and C Streets NW.
• Moving the Department of Commerce site from the National Mall to between 14th and 15th Streets NW between C and B Streets NW.
• Moving the National Archives site northward to between 12th and 13th Streets NW and Pennsylvania Avenue NW and C Streets NW (cutting off D Street NW).
• Retaining the previously announced site of the Internal Revenue building.

The Public Buildings Commission also announced it would build an 1,800-car parking lot next to the Department of Commerce building, and would proceed with construction of the Commerce and Archives first (as they were the top priority). Three months later, the estimates for construction of the Commerce building was increased to $16 million from $10 million and for the Internal Revenue building to $10.5 million from $2.5 million. Work on the Commerce building site was expected to begin by March 31, 1927. Government officials, other experts, and the press believed that the demolition of the District Building and Old Post Office Pavilion and the closure of many streets in the area would occur.

Work on all buildings was postponed in May 1927. On May 6, an ad hoc committee composed of Olmsted; Medary; Charles Moore, chair of the Commission on Fine Arts; and Louis E. Simon, Supervising Architect of the U.S. Treasury, recommended relocating the Justice building from 15th Street NW to a lot further east so that traffic congestion at 15th and Pennsylvania might be alleviated. This ad hoc committee met again three days later to not only consider the Justice building relocation but also to consider a plan to create a single building ringing Federal Triangle rather than six to eight individual structures. The Public Buildings Commission considered the same plan on May 16. The Commission on Fine Arts approved relocating the Justice building the following day. However, disagreements among the three planning bodies proved so fundamental that a new Board of Architectural Consultants was created on May 19, 1927, to advise the groups on the development of Federal Triangle. The Board consisted of the Supervising Architect of the U.S. Treasury (Louis E. Simon) and six private architects, including Louis Ayres, Edward H. Bennett, Arthur Brown Jr., William Adams Delano, Milton Bennett Medary, and John Russell Pope.

The Board of Architectural Consultants first met on May 23, at which time it considered a plan to create a single building ringing Federal Triangle rather than six to eight individual structures. As the Board of Architectural Consultants began its deliberations, the Commission on Fine Arts approved a plan to locate the Justice building on the north side of B Street NW between 7th and 9th Streets NW (where Center Market stood). About two weeks later, the Public Buildings Commission approved the single structure plan. This plan envisioned a central plaza (defined by 13th, 14th, B, and D Streets NW) surrounded by a traffic circle, with the buildings lining the exterior of the traffic circle. Few streets would be closed; rather, arches would connect each building to its neighbors (with only 12th Street NW remaining unbridged).

The final design of Federal Triangle began to come together in June 1927. The Board of Architectural Consultants approved the construction of the Commerce and Internal Revenue structures as stand-alone buildings on the sites last proposed in late June. In July, the Board proposed eight buildings, sited as follows:

• Archives (surrounded by Interstate Commerce on the north, east, and south)
• Commerce (west side of 15th Street NW between B and D Streets NW)
• General Accounting (13th, 14th, B, and C Streets NW)
• Independent Offices (6th, 7th, and B Streets NW and Pennsylvania Avenue NW)
• Internal Revenue (B, 10th, 12th, and C Streets NW)
• Interstate Commerce (9th, 10th, and B Streets NW and Pennsylvania Avenue NW)
• Justice (7th, 9th, and B Streets NW and Pennsylvania Avenue NW)
• Labor (B, 13th, 14th, and C Streets NW)

The Board did not address the future of the District Building, Old Post Office Pavilion, or Southern Railway Building, but had tentatively agreed to continue with the "Louvre plan" of a ring of buildings joined by arches.

The first design contract for any of the buildings in Federal Triangle was statutorily required by Congress in 1926 as part of the Public Buildings Act. A new headquarters for the Department of Commerce had been proposed in 1912 and a contract for the design work awarded to the architectural firm of York and Sawyer. Although this building was never built, Congress honored the contract and named the firm again as the Commerce building's designer. By March 1927, government officials had already decided that the Commerce building should be 1,000 feet (305 metres) long—making it the then-largest building in the District of Columbia. The May 1927 work moratorium, however, put these plans on hold. In September 1927, the design of the Internal Revenue building was handed over to Louis Simon at the Treasury Department, and the Commission of Fine Arts met to discuss proposed plans for both the Commerce and Internal Revenue buildings. At the same time, the commission received bids on demolition of existing structures in the Triangle.

After review by the Board of Architectural Consultants, the Public Buildings Commission gave final design approval on November 1, 1927, to the Commerce and Internal Revenue buildings. The previous sizes of both buildings was reaffirmed, as was the "Louvre plan" for a unified ring of buildings surrounding a traffic circle and plaza. The Commission on Fine Arts adopted a requirement that the planned Federal Triangle buildings have a "uniform appearance" and height (six stories), limiting the Board's deliberations. Secretary Mellon imposed a requirement that all the buildings be built in the Neoclassical architectural style. By mid-December 1927, the design of the Archives building had been approved, and the Board of Architectural Consultants was meeting again to study once more the general layout of the Federal Triangle.

By March 1928, newspapers had reported that the Commerce and Internal Revenue buildings would be constructed first, followed by the Archives, then Justice, and then a newly added Post Office building. Plans continued for the demolition of the District Building and Southern Railway headquarters (although the latter would be the last to be razed, as it would be used as temporary office space for displaced federal workers). Although the Commerce building plans (a 1,051-foot-long [320 m] building with 1,000,000 square feet [93,000 m] of office space, the largest office building in the world) had stabilized by March 1928, some designers suggested that both 15th and 14th Streets NW be submerged in tunnels beneath the structure. About the same time, the Internal Revenue building's square footage was reduced by almost a quarter to 500,000 square feet (46,500 square metres). In July, Congress appropriated $210,000 for design work for the Independent Offices, Interstate Commerce, Justice, and Labor buildings, and Secretary Mellon altered the work schedule yet again to focus on these structures. The Board of Architectural Consultants met to consider ways in which the construction program might be sped up, and devised plans to have four approved buildings (Commerce, Internal Revenue, Justice and Labor) completed by 1932. By October 1928, the Board of Architectural Consultants had agreed with prior decisions that no office building should be constructed on the National Mall, and that this space should be reserved for museums.

Plans for the eastern apex of Federal Triangle, however, were complicated by an ongoing effort to create a George Washington Memorial. A George Washington Memorial Association was organized in 1898 to establish in the District of Columbia a university bearing Washington's name. Efforts to do so were unsuccessful, but in 1904 the Association signed an agreement with D.C.-based Columbian University to change its name to George Washington University and build a large memorial hall on the university's campus. Plans for the memorial hall did not move forward, however, so the Association joined with the Smithsonian Institution to build a similar structure on the former site of the Baltimore and Potomac Railroad's Pennsylvania Station. A design competition was held in 1914, and architects chosen. The cornerstone was laid in 1921, and some of the foundation and a marble stairway built on a plot of land across B Street NW (where the National Gallery of Art sits today) in 1924. In 1929, even as the Federal Triangle project was moving forward, the George Washington Memorial Association was conducting fund-raising for the construction of the building at the proposed National Archives site. Press reports, however, indicated that the building had already been displaced from the Apex building site. The fund-raising effort eventually failed, and the foundation and stairs were razed in 1937 to make way for the National Gallery of Art.

Architectural models of the proposed Federal Triangle development were unveiled in late April 1929. Design work on the Independent Offices, Justice, and Labor buildings also began at that time. After these models were unveiled, however, the Board once more made changes to the Federal Triangle construction plan to reflect the March and April changes made by Hoover and Mellon. Now only seven large structures were planned, and assigned to the following board members for design:

• Apex Building (formerly the Independent Offices building, and now assigned to house the United States Coast Guard) - Bennett
• Commerce Department building - Ayers (of the firm York and Sawyer)
• Internal Revenue Service building - Simon
• Justice Department building - Medary
• Labor Department/Interstate Commerce Commission (ICC) building, and Departmental Auditorium - Brown
• National Archives building - Pope
• Post Office Department building - Delano

Two major changes to the complex came in early 1930. The Board and other planning groups had long agreed to site the Justice Department building on the block bounded by 7th, 9th, and B Streets NW and Pennsylvania Avenue NW. But this plan changed in March 1930. Architect John Russell Pope made a proposal to have the Justice and Archives switch sites so that the Justice building would have more space. Although the change would entail major design alterations in both buildings, Secretary Mellon favored the idea. The Commission on Fine Arts approved the plan, and Mellon met with the Board of Architectural Consultants in late March 1930 to discuss the idea. Although this initial meeting left the issue unresolved, the Board later agreed to Mellon's wishes in April and the two buildings switched plots. At the end of April, President Hoover asked Congress to appropriate $10.3 million (the most yet) to build a new Post Office Department building between 12th and 13th Streets NW, from Pennsylvania Avenue NW south to C Street NW.

Parking and traffic issues proved immensely vexing for the planners of Federal Triangle. The original L'Enfant Plan setting out the streets of Washington, D.C., still existed in the Federal Triangle area. Both C Street NW and D Street NW still ran from 15th Street NW to 15th Street NE. Ohio Avenue NW ran in a northwest–southeast line from the intersection of D and 15th Streets NW to the intersection of B and 12th Streets NW (soon to be renamed as Constitution Avenue NW and 12th Street NW). Louisiana Avenue NW still ran in a southwest–northeast direction from 10th and B Streets NW to 7th and D Streets NW (along what is currently the diagonal portion Indiana Avenue NW). The McMillan Plan was developed before the widespread use of the automobile, and now the Board of Architectural Consultants had to decide how to accommodate the "horseless carriage" while also making Federal Triangle pedestrian-friendly. The Board began studying traffic issues in late 1927. A major study of parking needs and solutions was conducted in 1931, and traffic and parking patterns assessed again after the Department of Commerce building opened in early 1932. To achieve some of the traffic and parking goals, the east–west streets and diagonal avenues were eliminated, leaving only the north–south streets through the area, and 12th and 9th Streets NW were submerged in tunnels beneath the National Mall. In the first major change to the Board's "final" plans, the Grand Plaza was abandoned in favor of a parking lot. The Board considered a number of other solutions to the need to accommodate the more than 7,500 cars expected to arrive every day (including an underground bus terminal and underground parking garage under the Grand Plaza), but in the end only approved a small number of underground parking spaces beneath the Apex Building.

The design of Federal Triangle was significantly influenced by the Louvre-Tuileries Palace complex in France and the concentration of government buildings in Whitehall in the United Kingdom. But planning for the complex was also deeply influenced by the City Beautiful movement and the idea of creating a civic center to achieve efficiency in administration as well as reinforce the public's perception of government as authoritative and permanent. For the architectural style of the buildings, the Board relied heavily on the McMillan Plan's recommendation of the Neoclassical style. Both the Board and Treasury Secretary Andrew W. Mellon rejected the Modern style then heavily in vogue. Rather than a mass of tall, imposing buildings, two unifying open spaces (intended for ceremonial use, and under discussion by the Board at least by March 1928) would be utilized. The first would be a Circular Plaza (inspired by the Place Vendôme) bisected by 12th Street NW, and which would require the demolition of the Old Post Office Pavilion. The second would be a rectangular Grand Plaza on the east side of 14th Street NW between the proposed Department of Commerce building (west side of 14th Street NW) and the proposed Post Office Department building (east side of 13th Street NW). The construction of the Grand Plaza would have required the demolition of the District Building.

The Board received significant input from the Commission of Fine Arts (which strongly advocated implementation of the McMillan Plan), National Capital Parks and Planning Commission, and the Washington Board of Trade. One guiding principle for the project was that office space for at least 25,000 federal workers must be included. Another was that, although the buildings would be modern steel frame structures, they would each be the same height and faced with limestone. The National Capital Parks and Planning Commission had the least influence over the overall design of the project, primarily because it had only recently been formed.

Although the Board unveiled its proposed design for the project in 1929, the design still lacked a unifying architectural look. Subsequently, John Russell Pope was asked in September 1929 to bring a more uniform style to the buildings. Nonetheless, within this more uniform approach, a variety of styles could be used, and were: Italian Renaissance for the Department of Commerce building, Corinthian for the National Archives building, and Ionic for the Post Office Department. Meanwhile, the Board worked with sculptors, painters, and others to design more than 100 statues, fountains, bronze doors, murals, plaques, and panels (both interior and exterior) throughout the complex. Modern architectural styles were not completely ignored in the design effort, however; most of the doors and grillwork throughout the Federal Triangle complex were Art Deco in style.

The U.S. Treasury Department signed a contract to raze the buildings on the site of the planned Internal Revenue Service building in October 1926. The length of the Commerce building was set at 1,000 feet (300 m) in March 1927, and survey work at the site began on March 31, 1927 (even though final plans for the project were still unclear). Work on all buildings was postponed in May 1927. Work began again in September 1927, with demolition work on the Commerce and Internal Revenue sites. Excavation of both sites began on November 21, 1927. Additional demolition contracts were awarded for both sites in April 1928, and foundation work for the Internal Revenue building began in June 1928. Some 8,000 pilings were driven into the soft ground to support the foundation. Work on Internal Revenue's superstructure began on March 8.

The cornerstone of the first building to be constructed, the Internal Revenue building, was laid by Treasury Secretary Andrew W. Mellon on May 25, 1929. While digging its foundation, workers uncovered a dock which was at least 100 years old. Indiana limestone was used for the facing, and Tennessee marble for the columns. The sites of all the Triangle buildings had been established in their revised positions by May 1929, with two exceptions: The positions of the Justice and Archives buildings remained as originally planned (with Justice in the east), and the apex space remained unallotted. President (and former Commerce Secretary) Herbert Hoover laid the cornerstone of the Commerce building on June 10, 1929, using the same trowel President George Washington had used to lay the cornerstone of the U.S. Capitol. The construction contract for the Commerce building (set at $13.567 million) had been signed in March, and the contract for its limestone facade—according to at least one newspaper account, the largest stone contract in world history—was awarded in April. By then, the cost of the Commerce building had risen to $17.5 million. Due to the formerly marshy condition of the soil and several submerged streams nearby, more than 18,000 pilings had to be set to support the Commerce building. Water pressure from the submerged Tiber Creek made it too difficult to drive the piles, so a deep-sea diver descended into the underground Tiber Creek and drilled a hole 20 feet (6.1 metres) deep into the earth. A hose was inserted into the hole, and water pumped from the earth until the water table dropped and the driving of the piles could be accomplished. The October 1929 stock market crash and subsequent Great Depression led newly elected President Herbert Hoover to increase spending on existing public works as a means of stimulating the economy. No funds had been authorized yet for the Archives, Independent Offices, Justice, or Labor buildings, and Hoover secured an additional $2.5 million a year for 10 years from Congress for this purpose. Work on the Justice and Independent Offices/Labor buildings now proceeded.

Treasury officials hoped to have the Post Office building under construction and ground broken for the Justice building by December 1930. But a major design change and funding choices were made in 1930. Pope convinced the Commission on Fine Arts to switch the positions of the Department of Justice and National Archives, giving the Justice building more space. Congress also amended the Public Buildings Act to permit private (not just federally employed) architects to bid on design contracts, and agreed to fund the construction of the Justice, Labor/ICC, National Archives, and Post Office buildings.

1931 saw much of the Federal Triangle project near completion. In March 1931, Congress appropriated $3 million to begin construction of the Apex Building, the last structure to be funded. Demolition began on the site the following November. The Internal Revenue building was completed and occupied in June. It was finished a year ahead of schedule, and contained more than 672,000 square feet (62,500 square meters) of office space (3.4 percent more than originally planned). But work had yet to begin on the Justice, Labor, or Post Office buildings. Work on the ICC building finally began in April 1931 when the two blocks for the site began to be razed, and 13th Street NW was permanently blocked off at that time. The first work on the Post Office site began in July with demolition as well. Demolition of existing structures on the Archives site ended in August 1931, and ground was finally broken on September 5. Meanwhile, razing of the ICC/Labor site was also completed by the first of September, and excavation work began shortly thereafter.

In 1932, the Commerce building opened and construction began on three additional buildings at Federal Triangle. The Department of Commerce building opened on January 4, 1932. The finished building had 1,605,066 square feet (148,271.1 square metres) of office space (more than 60 percent larger than originally planned), and its foundation was more than three feet thick in places in order to withstand the hydraulic pressure put on it by the submerged Tiber Creek. Water from the Tiber was utilized as an air conditioning system, to cool the building. July saw the construction contract for the $7.67 million Justice Department building signed. Although some funds for Federal Triangle projects had not been appropriated yet, work still went ahead using funds from other projects which were behind schedule. On September 26, 1932, the 143rd anniversary of the founding of the U.S. Postal Service, President Hoover laid the cornerstone of the Post Office Department building (although the foundation had already been laid, and the steel superstructure of the building was already three stories high). Once again, Hoover used the trowel which George Washington had used to lay the Capitol's cornerstone. Congress had appropriated $10.3 million for the new structure, which was designed to accommodate more than 3,000 workers. Its eight stories would be laid on a granite foundation and the sides clad in limestone. On December 1, 1932, the contract for construction of the limestone National Archives building (whose estimated construction cost was $5.284 million) was awarded to the George A. Fuller Company (which had constructed the New York Times Building and the Flatiron Building). Hoover laid two cornerstones on December 15 for the Labor/ICC building, the third building to begin construction that year. Workers who were Freemasons assisted the President in laying the cornerstones. Hoover personally oversaw the dedication of the cornerstone at the Labor end of the building. His words were broadcast over loudspeaker to the workers at the ICC end of the structure, who placed the ICC cornerstone simultaneously at the President's instruction (becoming the first time in Washington history that a single person dedicated two cornerstones at the same time). William Green, President of the American Federation of Labor, attended the laying of the cornerstone for the Labor building. Once more, Hoover used the trowel used by Washington to lay the cornerstone of the Capitol. Two weeks later, on December 30, the concrete foundation of the Archives building was poured.

Two days before George Washington's birthday, U.S. President Herbert Hoover laid the cornerstone of the National Archives building on February 20, 1933. The structure's cost was set at $8.75 million. Just three days later, he laid the cornerstone of the Department of Justice building with a trowel made from wood and cooper nails from the USS Constitution. Chief Justice Charles Evans Hughes, Associate Justice Harlan Fiske Stone, Associate Justice Owen Roberts, Associate Justice James Clark McReynolds, Solicitor General Thomas D. Thacher, and Attorney General William D. Mitchell all attended the ceremony. Five months later, a small fire at the Post Office construction site was extinguished by a security guard before it could do any damage. Late in 1933, the northern addition to the Internal Revenue building (the land had been condemned in December 1930) began to rise.

Only one building remained to be constructed under the new administration of President Franklin D. Roosevelt. By December 1933, the President was preparing his proposal to Congress for full funding of the Apex Building construction project. But the Apex Building itself almost was not built, as Pope and others argued that it tended to hide the planned National Archives building. Still others thought the site should be used for the proposed Jefferson Memorial. Through Roosevelt's personal intervention, the building was saved—but nearly all its external ornamentation was stripped, and plans for a terraced fountain nearby eliminated, although a small fountain was built in what eventually became known as Patrick Henry Park. Construction of the building was re-approved on January 18, 1934. The building's final cost was estimated at just over $12 million.

In 1934, one building began construction and another finished. The Apex Building site began to be cleared in April. The Post Office building was occupied on May 6, and Postmaster General James Farley dedicated the Post Office Department building on June 11, 1934. Its final cost was $10.83 million, about half a million dollars over budget. Construction in the rest of the Triangle, however, seemed stalled. Although some structures had been razed on the site, no appropriation had been made for the Apex Building. The government had also cleared land northeast of the Internal Revenue building (as planners considered adding yet another building to the Triangle), but Congress was increasingly opposed to demolishing either the Old Post Office or the District Building. City officials considered selling the District Building to the federal government as a means of raising cash to build a new city hall, but federal officials balked at the idea. President Roosevelt dedicated the newly opened Department of Justice building on October 25, 1934. Chief Justice Hughes, all the Associate Justices of the Supreme Court, Attorney General Homer Stille Cummings, and a large number of foreign ambassadors also attended the dedication. The $11 million structure had more than 550,000 square feet (51,200 square meters) of office space. By November 1934, the addition to the Internal Revenue building was nearing completion, and government officials were contemplating the razing of the District Building, Old Post Office Pavilion, and Southern Railway Building.

The Labor/ICC and Archives buildings were opened in 1935. Numerous strikes (see below) had delayed the opening of the building for almost a year. In April 1934, Secretary of Labor Frances Perkins asked for additional (if minor) design changes. Pressing needs for office space meant that a portion of the ICC building was occupied before the structure was finished. Minor alterations were made to the Labor building (such as creating a private rather than shared bathroom for the female Secretary) in January 1935. Secretary of Labor Frances Perkins dedicated the two buildings at a ceremony in the Departmental Auditorium on February 26, 1935, attended by AFL President William Green. The Labor building's final cost was $4.5 million and the ICC portion of the structure cost $4.45 million. The two buildings were connected by the 2,000-seat Departmental Auditorium (renamed the Andrew W. Mellon Auditorium in 1987). Meanwhile, construction forged ahead on the Archives building. Already considered too small to hold all the materials in its possession, a proposal had been made to add another story to the building. This proposal was rejected in March 1935. The National Archives building was occupied in November 1935, but had no formal dedication. Although the Archives structure had been one of the top priorities of almost all planners, it was one of the last buildings to be opened. Archives staff began moving into the building in November 1935, and the exhibition rotunda was opened to the public in November 1936. Records were not transferred in large numbers to the building until April 1937.

The Apex Building was the last to be constructed and dedicated. A major impetus for the building's construction came in June 1935, when the Federal Trade Commission's (FTC) headquarters at C Street NW and 21st Street NW was razed to make way for the Federal Reserve Board Building. The FTC petitioned the Commission on Fine Arts to permit it to occupy the Apex Building. Testing for the foundation began in September 1936, and was completed shortly thereafter. With design work long completed and President Roosevelt expressing his wish that the structure be built, a $3.1 million contract for the building was signed on December 29, 1936. Using the George Washington trowel, President Roosevelt laid the cornerstone for the building on July 12, 1937. The building's $3.665 million cost was paid for out of Public Works Administration funds, and officials estimated it would be ready for occupation by January 1, 1938. Demolition of the old foundation for the unbuilt Washington Memorial occurred in July 1937, and much of the Apex Building's steel superstructure was rising by then as well. The great depth of the building's foundation meant that the crane operator lifting the steel beams into place was out of sight in the basement, and a series of telephone links from observers on the street relayed instructions to him. By December 1937, the building was two months ahead of schedule.