The Grand Ballroom at New York’s storied Plaza Hotel, reopening Wednesday after a $12.5 million restoration, has served as the backdrop to countless affairs — including such star-studded gatherings as Truman Capote’s “Black and White Ball” and Mick Jagger’s 50th birthday party; the opulent neoclassical room has rightfully earned a place in the annals of the city’s beau monde history. So when developer Elad Properties bought the hotel in 2004 and authorized a $400 million top-to-bottom makeover that would convert part of the building to private residences, the restoration and renovation of the 4,800-square-foot Grand Ballroom received top billing.

With the first post-restoration event, a party for Chanel Fine Jewelry on Wednesday, the legendary space is poised to reprise its long-established role of housing many of New York’s most exclusive fêtes. To ensure that the illustrious reputation of the ballroom remain intact, Elad Properties enlisted the expertise of (and signed a 25-year ballroom lease with) Events at The Plaza — a joint venture between an upscale events and catering company, Great Performances of New York and a provider of hospitality and food services, Delaware North Companies of Buffalo.

For project architect Richard Bloch, one of the major challenges of the restoration was the ballroom’s landmark status, which restricts alterations that may be made to it. (In addition to the ballroom, its chandeliers, and 16 sconces, the Plaza’s Palm Court, Oak Bar, Terrace Room, Fifth Avenue entrance, and other portions of the building have been designated city landmarks.) “Essentially our role is to monitor the work to ensure that it is faithful to what was originally there,” a spokeswoman for the Landmarks Preservation Commission, Elisabeth de Bourbon, said.

The commission worked closely with Mr. Bloch to achieve a restoration that took into account both how the space looked in 1929 and the need for the modern amenities and safety-related upgrades that a first-rate catering and events hall needs.

The space’s original decor became known as the “Plaza Style,” — an adaptation of the French Renaissance Revival style, replete with attached Ionic columns, a coved ceiling and an overarching palette of white and gold. To bring back that aesthetic, the required alterations included replacing and repainting the wall plaster; replacing the carpet, and restoring four historic murals, the stage and balconies, with their curved ornamental railings, and the chandeliers and sconces.

Mr. Bloch relied heavily on the findings of a two-year-old report drawn up by Jablonski Building Conservation Inc. The report detailed original finishes of the Plaza’s historic spaces, including the intricacies of the ballroom’s original interior decoration, which was designed and constructed by the architecture firm of Schultze and Weaver. “We made some wonderful discoveries,” Mr. Bloch said. “We dug deep into the metal and found that the balcony railings were originally a white metal.”

While the color was previously identified as shimmery silver, Mr. Bloch and his crew, tipped off by the Jablonski report, discovered that the railings had been a much lighter hue. The color of the gilding also required some investigation. After a series of meticulous paint studies, it was decided that the original color was not as brilliant as first thought. The new gilding would be painted a luminous gold color, but then it would be covered with a glaze for a more muted look.

The wall color was essentially white, but it was difficult to discern the original shade. “Because of the limits of black-and-white photography we did not know the exact color of the walls,” Mr. Bloch said. “But, we worked with Landmarks and got it almost exactly the way it was, a rich cream color with a slight shift.”

Mr. Bloch said that there were other instances in which the original color or style was identified, but Landmarks approved a “slight shift” based on what modern audiences would like and what would work best in the space.

Originally, two chandeliers had lit the entire space. As part of the renovation, though, the chandeliers were rewired to power more than 100 light bulbs, thus bringing the room up to current lighting codes. But in the spirit of the original space, Bloch hid the wires in a sunken trench surrounding the perimeter of the room. As for the chandelier’s crystals, all 2,000 of them were individually cleaned, and damaged crystals were replaced with those in better condition.

To address potential safety issues, the number of guests that the ballroom can accommodate has been greatly reduced in the new space to 450 people — from 1,000. The flow of traffic between the kitchen and the ballroom has been refigured to improve the room’s efficiency as a catering space. And to further enhance the room’s capabilities, completely new additions were constructed, including a bridal suite, a pre-function space, new bathrooms, and a new state-of-the-art kitchen.

The result of this exhaustive effort will be tested this week, when the Grand Ballroom’s glimmering chandeliers illuminate the space, once again.