Casey returned to Washington, D.C. in October 1940 to become chief of the Design and Engineering Section in the Construction Division of the Office of the Quartermaster General, under Brigadier General Brehon B. Somervell.
An enormous construction program was underway to meet the needs of World War II.
Working with a staff that included George Bergstrom, a former president of the American Institute of Architects, Casey set about revising the standard designs for barracks.
A number of new features were added to improve comfort, safety, and durability.
Substitutions were made for scarce materials.
It was discovered that the standard 63-man barracks was now too small.
Of the 81 companies in the new triangular division, 51 fitted more easily into 74-man barracks.
By slightly increasing the barracks size, substantial savings were made by reducing the overall number of buildings that needed to be constructed, the size of the cantonment areas required, and the length of required roads and utility lines.
Casey was promoted to lieutenant colonel on 8 April 1941.

On the afternoon of Thursday, 17 July 1941, Somervell summoned Casey and Bergstrom and gave them a new special project: the design of an enormous office complex to house the War Department's 40,000-person staff together in one building.
Somervell gave them until 09:00 on Monday morning to design the building, which he envisaged as a modern, four-story structure with no elevators, on the site of the old Washington Hoover Airport.
 This would ultimately become The Pentagon, the largest office building in the world.
 Over that "very busy weekend", Casey, Bergstrom and their staff roughed out the design for a four-story, five-sided structure with a floor area of 5,100,000 square feet (470,000 m²)—twice that of the Empire State Building.
The estimated cost was $35 million.
President Roosevelt subsequently moved the site of the building, over Somervell's objections, away from Arlington National Cemetery.
Casey returned to Washington, D.C. in October 1940 to become chief of the Design and Engineering Section in the Construction Division of the Office of the Quartermaster General, under Brigadier General Brehon B. Somervell. An enormous construction program was underway to meet the needs of World War II. Working with a staff that included George Bergstrom, a former president of the American Institute of Architects, Casey set about revising the standard designs for barracks. A number of new features were added to improve comfort, safety, and durability. Substitutions were made for scarce materials. It was discovered that the standard 63-man barracks was now too small. Of the 81 companies in the new triangular division, 51 fitted more easily into 74-man barracks. By slightly increasing the barracks size, substantial savings were made by reducing the overall number of buildings that needed to be constructed, the size of the cantonment areas required, and the length of required roads and utility lines. Casey was promoted to lieutenant colonel on 8 April 1941.
On the afternoon of Thursday, 17 July 1941, Somervell summoned Casey and Bergstrom and gave them a new special project: the design of an enormous office complex to house the War Department's 40,000-person staff together in one building. Somervell gave them until 09:00 on Monday morning to design the building, which he envisaged as a modern, four-story structure with no elevators, on the site of the old Washington Hoover Airport.  This would ultimately become The Pentagon, the largest office building in the world.  Over that "very busy weekend", Casey, Bergstrom and their staff roughed out the design for a four-story, five-sided structure with a floor area of 5,100,000 square feet (470,000 m²)—twice that of the Empire State Building. The estimated cost was $35 million. President Roosevelt subsequently moved the site of the building, over Somervell's objections, away from Arlington National Cemetery.
Hugh John Casey