As it had following the 1917 work, the courthouse continued to serve its purpose for the next three decades.
By 1999, however, it once again seemed inadequate to the needs of its users.
The thick walls made it difficult to adapt for modern electrical and telecommunications purposes.
The infrastructure within was almost half a century old, making it difficult to heat and cool the building.
The space taken up by those walls meant that the courtroom was the only place the judges and all their combined staff could hold meetings.
There were also very few spaces for the public within the building, frustrating attempts to make the court and history transparent and accessible.

The judges decided that it was necessary to make some small additions to the building in addition to the infrastructure upgrades.
Chief Judge Judith Kaye designated Judge Richard C. Wesley, later elevated to the federal Second Circuit Court of Appeals, as the court's liaison to the project, now managed by the Dormitory Authority of the State of New York as the state architect's position had been abolished.
After the necessary appropriations from the legislature, work began in 2001.
During the 17 months it took, the judges continued to hear arguments in the courtroom but worked out of temporary quarters outside the city.
Project architects were DeWolff Partnership Architects LLP.

Since the building had been listed on the National Register of Historic Places since its last renovation, preservation and restoration were as important as the expansions.
The State Historic Preservation Office was closely involved with the process.
It mandated that the newly installed wings on the sides of the building be distinguishable from the original facades after the marble was once again replaced with newly quarried Vermont stone, even though they were architecturally sympathetic to Rector's original design.
As a result, they have narrower bays and pilasters, smooth at the base rather than fluted, to subtly distinguish them from the original building.

Elsewhere on the exterior, the dome was made over.
The lead-plated copper installed in 1959 was replaced with stainless steel and the cupola from that renovation removed, to be replaced with the current oculus.
High-performance glass in it keeps out ultraviolet light and reduces heat inside.
The gold-plated sphere that had topped the cupola remains above the oculus.

Inside, the renovations once more made the ceiling groin vaults visible.
The entire infrastructure was replaced, and the newly created space in the main building, plus the additional space in the wings, brought the building to 93,000 square feet (8,600 m²) of total floor space.
The Red Room and the Jay Room were created on the first floor to host public functions and exhibits on the history of the court, as well as providing overflow space for those who wished to hear oral arguments in cases that attracted significant interest.
All the judges' chambers were now located on the second floor, a change that Kaye had particularly sought.

Photographs and other historical records were studied closely for information about interior decoration that matched earlier eras.
The rotunda was done in shades of beige, cream and yellow, as it had appeared to be originally.
On the first floor carpets, blues and reds were used following early 20th-century practices.
The Red Room and all offices similarly have a painted faux-finish on the walls.
Since the third floor is now largely new construction, it uses green primarily.
Lighting used throughout duplicates the 1959 renovation, except in the courtroom, where reconstructions of the 1884 chandeliers complement a newly designed carpet.
As it had following the 1917 work, the courthouse continued to serve its purpose for the next three decades. By 1999, however, it once again seemed inadequate to the needs of its users. The thick walls made it difficult to adapt for modern electrical and telecommunications purposes. The infrastructure within was almost half a century old, making it difficult to heat and cool the building. The space taken up by those walls meant that the courtroom was the only place the judges and all their combined staff could hold meetings. There were also very few spaces for the public within the building, frustrating attempts to make the court and history transparent and accessible.
The judges decided that it was necessary to make some small additions to the building in addition to the infrastructure upgrades. Chief Judge Judith Kaye designated Judge Richard C. Wesley, later elevated to the federal Second Circuit Court of Appeals, as the court's liaison to the project, now managed by the Dormitory Authority of the State of New York as the state architect's position had been abolished. After the necessary appropriations from the legislature, work began in 2001. During the 17 months it took, the judges continued to hear arguments in the courtroom but worked out of temporary quarters outside the city. Project architects were DeWolff Partnership Architects LLP.
Since the building had been listed on the National Register of Historic Places since its last renovation, preservation and restoration were as important as the expansions. The State Historic Preservation Office was closely involved with the process. It mandated that the newly installed wings on the sides of the building be distinguishable from the original facades after the marble was once again replaced with newly quarried Vermont stone, even though they were architecturally sympathetic to Rector's original design. As a result, they have narrower bays and pilasters, smooth at the base rather than fluted, to subtly distinguish them from the original building.
Elsewhere on the exterior, the dome was made over. The lead-plated copper installed in 1959 was replaced with stainless steel and the cupola from that renovation removed, to be replaced with the current oculus. High-performance glass in it keeps out ultraviolet light and reduces heat inside. The gold-plated sphere that had topped the cupola remains above the oculus.
Inside, the renovations once more made the ceiling groin vaults visible. The entire infrastructure was replaced, and the newly created space in the main building, plus the additional space in the wings, brought the building to 93,000 square feet (8,600 m²) of total floor space. The Red Room and the Jay Room were created on the first floor to host public functions and exhibits on the history of the court, as well as providing overflow space for those who wished to hear oral arguments in cases that attracted significant interest. All the judges' chambers were now located on the second floor, a change that Kaye had particularly sought.
Photographs and other historical records were studied closely for information about interior decoration that matched earlier eras. The rotunda was done in shades of beige, cream and yellow, as it had appeared to be originally. On the first floor carpets, blues and reds were used following early 20th-century practices. The Red Room and all offices similarly have a painted faux-finish on the walls. Since the third floor is now largely new construction, it uses green primarily. Lighting used throughout duplicates the 1959 renovation, except in the courtroom, where reconstructions of the 1884 chandeliers complement a newly designed carpet.
New York Court of Appeals Building