The Felix M. Warburg House is a mansion at 1109 Fifth Avenue, on the Upper East Side of Manhattan in New York City.

When plans to replace the mansion with luxury apartments fell through, ownership of the house reverted to the Warburgs, who then donated it in 1944 to the Jewish Theological Seminary of America.

The mansion was designed in the Châteauesque style by C. P. H. Gilbert and retains its original facade, characterized by French Gothic details around the windows and on the roof line.

The interior of the Warburg House, wholly occupied by the Jewish Museum, has a total floor space of 82,000 square feet (7,600 m2).

[10][11] In turn, Schiff was the head of the New York–based banking house Kuhn, Loeb & Co.,[11] which Warburg had joined as a junior partner in 1897.

[13] The Warburgs had four children by 1907 and, needing space,[11][14] Frieda purchased a lot at the northeast corner of Fifth Avenue and 92nd Street from Perry Belmont.

[1][17] To design a new residence on their lot, the Warburgs hired the architect C. P. H. Gilbert,[10][18] who was at that time building a house for Felix's brother, Paul,[1] and had impressed the family with the mansion he built for Isaac D. Fletcher on Fifth Avenue.

[10][20] By the next month, workers were excavating the site, and Gilbert had hired Barr, Thaw & Fraser Co. to supply limestone for the mansion.

These included the wedding of their daughter Carola in 1916, which was attended by 900 guests;[27] a "dramatic reading" to raise money for World War I relief in 1918;[28] and a fundraiser for Jewish charities in 1928.

[38] In May 1941, she sold the mansion to developer Henry Kaufman and architect Emery Roth, who intended to redevelop the site into an eighteen-story apartment building.

[44] Percival Goodman was hired to redesign the mansion, and the JTS filed plans with the DOB to convert the building into a museum in September 1944.

[45] The renovations were delayed by World War II and, in December 1945, the seminary's president Louis Finkelstein announced that work would start immediately.

[50][51] Frieda Warburg said that, when she re-entered the house for the first time after its renovation, "I discovered to my joy that instead of depressing me, it gave me a wonderful feeling of happiness.

[59] Upon the completion of this wing, the museum's main entrance was relocated to the List Building, and the ground-story windows of the Warburg House were blacked out.

[70] The JTS again opposed designation, arguing that it would prevent the museum from modifying the mansion without the LPC's permission and significantly increase the cost of maintenance.

[63][73] Manhattan Community Board 8, representing the surrounding neighborhood, voted in November 1981 to recommend that the LPC not designate the building as a landmark.

[70] Subsequently, seven local groups and 70 preservationists formed the Alliance to Preserve the Warburg Mansion, which circulated a petition opposing the tower.

[65][79] The Jewish Museum agreed to relocate to the New-York Historical Society building for the duration of the project,[80][81] which ultimately cost $36 million.

[82] The work included completely reconstructing the List Building and transforming its interior into a 232-seat auditorium,[83][84] enlarging the museum's gross floor area from 52,300 to 82,000 square feet (4,860 to 7,620 m2), and moving its main entrance to 92nd Street.

The renovation, designed by Tsao & McKown Architects, involved removing a staircase and unsealing some windows that faced west toward Central Park.

The fifth and sixth stories contain dormer windows that project from a steeply sloped mansard roof, which is clad with slate tiles.

[6] The house's main entrance is within a projecting frontispiece on 92nd Street, which contains a depressed elliptical arch at ground level, above which is a balcony with a balustrade.

The music room had walls decorated with tapestries; wrought iron chandeliers suspended from beams in the ceiling; a fireplace mantel; and some display cases with rare books.

The second floor also contained a formal dining room with tapestries, upholstered chairs, and mantelpiece, along with a Gothic-style conservatory, where a small painting of Madonna with Child by Botticelli was displayed.

[99][94] A study was placed in the corner of that story, directly above the sitting room,[94] and the fourth-floor hallway contained wind-up toy train tracks.

[79] In 1909, after the Warburg House was completed, the Real Estate Record and Guide described the building as one of "a number of palatial residences" along Fifth Avenue.

[102] Christopher Gray wrote in 2004 that the mansion resembled the Isaac D. Fletcher House, "although it edges toward a simpler expression, with somewhat less detail".

[18][79][83][100] When the plans for the annex were first announced, members of the Municipal Art Society expressed both satisfaction and displeasure over the new design.

Some members praised it as a "modest and appropriate" addition complementing the original mansion, but others said the annex was "unimaginative and does nothing to show the evolution of design in our time".

[18][83][100] Benjamin Forgey of The Washington Post wrote: "This pleasing if unexciting design is surprising mainly because of who did it", since Roche was better known as a modern architect.