During this period Simon served on the board which planned the Federal Triangle, and obtained for the office the design of the Internal Revenue Service Building, construction of which began in 1928 and was completed in 1931.
[2] Though the office under Simon was responsible for the design and construction of many large buildings, he is often remembered for the introduction of standard, easily extendible plans which could be quickly executed for smaller communities.
Both large and small buildings could also be designed by one of the many in-house consulting architects, including Howard Lovewell Cheney, Thomas Harlan Ellett, William Dewey Foster, Eric Kebbon, Lorimer Rich, Rudolph Stanley-Brown and Gilbert Stanley Underwood.
A few of these competitions were carried out, including for Covington, Kentucky, and Leavenworth, Kansas, but the build up to World War II prevented its full adoption.
Works of the office during this latter period included the terminal of Washington National Airport, designed primarily by Howard Lovewell Cheney and completed in 1941.
[2] Outside of his regular duties, President Franklin D. Roosevelt selected Simon to be principal architect of his presidential library on the grounds of his Springwood estate in Hyde Park, New York.
Construction on the building, designed by Simon in association with consulting architect Henry J. Toombs in Roosevelt's preferred Dutch Colonial Revival style, began in 1939 and was completed in 1941.