The surprise turnaround by the National Capital Planning Commission (NCPC)reverses an earlier decision to reject the British architect's proposals for a massive glazed roof at the museum's Old Patent Office building.
Work on the steel frame for the canopy - similar to Foster's Great Court canopy at the British Museum - was already well under way in Germany when the commission decided to knock back the scheme in June.
Despite twice approving the project, the commission felt the new canopy would 'further degrade the character' of the existing Greek revival-style building and was demanding the Smithsonian rebuild the original courtyard - which the institution controversially flattened in 2003.
However, the (NCPC) has now approved the final site and building plans thanks to a number of minor alterations, including the introduction of low-iron glazing to reduce the 'greenish tint'.
The Smithsonian has also agreed to reconstruct a staircase 'to restore the architectural integrity of the portico' on the building's south façade and reinstall two courtyard fountains.
Designated as a National Historic Landmark, the building was rescued from demolition in 1958 by President Eisenhower, who transferred the property to the Smithsonian Institution for use as the National Portrait Gallery and the Smithsonian American Art Museum.