With the Mayflower Hotel finished but not yet furnished in September 1924, plans were made to enlarge the structure even before it opened.
The new owners perceived high demand for guest room suites, and quickly designed a $1 million ($14,578,748 in 2019 dollars) addition.
Construction began in October 1925, and within six weeks the 40-foot (12 m) deep foundation had been dug.
The addition opened on May 31, 1925.

The most prominent features of the Annex were the Presidential Suite and the Vice Presidential Suite.
The Presidential Suite occupied the 10th floor, and was decorated in green and gold in the Italianate style.
The Vice Presidential Suite occupied the ninth floor, and was decorated in dull and bright yellow in the Louis XVI style.
Each suite had 13 rooms, which included a foyer, drawing room, library, secretary's room, dining room, and five bedrooms—each with its own bath and kitchenette.
Each suite also had a maid's room, with an attached bath.
The furnishings of both suites were copies of museum pieces.
The Presidential Suite featured a marquetry table with ormolu fittings; a Louis XVI cabinet with painted panels; Oriental rugs; bronze and marble urns in the Neoclassical style; drapes of silk damask; and underdrapes of silk taffeta.
The suite's dining room featured Queen Anne style furniture.
The Vice Presidential Suite featured a dining room with furniture in the Sheraton and Hepplewhite styles.
Dining room furniture in both suites was manufactured from satin-walnut, and featured painted decorations and marquetry.
The bedrooms in both suites featured Louis XVI-, Adam-, and Federal-style furniture made of satinwood, walnut, and mahogany.
Each piece was painted, lacquered, or marquetried.
Dust-covers for the beds were also of taffeta.
Sofas and chairs in each suite were upholstered in imported brocades, while the walls were covered in hand-made tapestries.
Each suite had numerous shaded lamps, porcelain and crystal art objects, and gilt mirrors.
Original oil and watercolor paintings as well as etchings and engravings—many of them by famous artists—decorated the suites.
Each suite's bathroom was completely tiled in white, with silver-plated fixtures for the sink and shower, an engraved glass shower door, and a Swiss shower.
The kitchens, too, were tiled in white, and contained an electric stove and oven, a Frigidaire refrigerator, silver tableware, complete porcelain table setting, and fine table linens.

The second through eighth floors of the Annex contained guest suites.
Each suite had five bedrooms, and each bedroom had its own bath.
The first floor of the Annex was occupied by the Mayflower Coffee Shop, a vastly expanded version of the highly popular but extremely small café located on the ground floor of the existing hotel.
The basement of the Annex occupied by a huge laundry, which served the hotel and annex.
With the Mayflower Hotel finished but not yet furnished in September 1924, plans were made to enlarge the structure even before it opened. The new owners perceived high demand for guest room suites, and quickly designed a $1 million ($14,578,748 in 2019 dollars) addition. Construction began in October 1925, and within six weeks the 40-foot (12 m) deep foundation had been dug. The addition opened on May 31, 1925.
The most prominent features of the Annex were the Presidential Suite and the Vice Presidential Suite. The Presidential Suite occupied the 10th floor, and was decorated in green and gold in the Italianate style. The Vice Presidential Suite occupied the ninth floor, and was decorated in dull and bright yellow in the Louis XVI style. Each suite had 13 rooms, which included a foyer, drawing room, library, secretary's room, dining room, and five bedrooms—each with its own bath and kitchenette. Each suite also had a maid's room, with an attached bath. The furnishings of both suites were copies of museum pieces. The Presidential Suite featured a marquetry table with ormolu fittings; a Louis XVI cabinet with painted panels; Oriental rugs; bronze and marble urns in the Neoclassical style; drapes of silk damask; and underdrapes of silk taffeta. The suite's dining room featured Queen Anne style furniture. The Vice Presidential Suite featured a dining room with furniture in the Sheraton and Hepplewhite styles. Dining room furniture in both suites was manufactured from satin-walnut, and featured painted decorations and marquetry. The bedrooms in both suites featured Louis XVI-, Adam-, and Federal-style furniture made of satinwood, walnut, and mahogany. Each piece was painted, lacquered, or marquetried. Dust-covers for the beds were also of taffeta. Sofas and chairs in each suite were upholstered in imported brocades, while the walls were covered in hand-made tapestries. Each suite had numerous shaded lamps, porcelain and crystal art objects, and gilt mirrors. Original oil and watercolor paintings as well as etchings and engravings—many of them by famous artists—decorated the suites. Each suite's bathroom was completely tiled in white, with silver-plated fixtures for the sink and shower, an engraved glass shower door, and a Swiss shower. The kitchens, too, were tiled in white, and contained an electric stove and oven, a Frigidaire refrigerator, silver tableware, complete porcelain table setting, and fine table linens.
The second through eighth floors of the Annex contained guest suites. Each suite had five bedrooms, and each bedroom had its own bath. The first floor of the Annex was occupied by the Mayflower Coffee Shop, a vastly expanded version of the highly popular but extremely small café located on the ground floor of the existing hotel. The basement of the Annex occupied by a huge laundry, which served the hotel and annex.
Mayflower Hotel