London–based Foster + Partners and Madrid–based firm, Rubio Arquitectura, will work alongside each other to renovate and refurbish the Hall of Realms (also known as Salón de Reinos), a historic ballroom dating back to the 17th century, as part of an extension project for the Museo del Prado in Madrid. The two firms won an international design competition held by the Spanish art institution out of a pool of 47 architect firms to carry out the roughly $32 million (€30 million) project.

The Hall of Realms was once a part of the Buen Retiro Palace, which now serves as an annex of the Museo del Prado, and was commissioned by King Philip IV and built in 1630 to serve as his secondary residence, according to the the museum. The space was inducted to the Spanish national art museum just a little over a year ago, and previously served as the Museum Del Ejercito, or Army Museum, until 2010.

The transformations include replacing the historical site's roof, and reorganizing the floors to create new gallery and communal spaces. A main area of focus within the historic building is its southern façade, which. To fully reveal the original form of the structure, multiple floor plates will be removed, which will also open up the area to create an indoor public plaza, dubbed the new Prado Campus. This space will be used for gatherings, dinners, recreation, and special events—hankering back to Philip IV's original vision. The facade's original openings will also be reestablished, allowing sunlight into the interiors and bringing back the edifice's relationship with the outdoors.

The loss of gallery space on the ground level, which will act as the heart of the building, will be made up for with a new, third floor, residing above the newly restored façade. The new roof for the added floor can be controlled to manipulate the amount of lighting allowed into the space, and will be powered by solar cells, while also providing shade along the southern side of the building, according to the Museo del Prado.

The building will be complete in 2019, coinciding with the museum's 200th anniversary.