The White House Reconstruction, also known as the Truman Reconstruction, was a comprehensive dismantling and rebuilding of the interior of the White House from 1949 to 1952. In 1948, after nearly a century and a half of wartime destruction and rebuilding, hurried renovations, additions of new services, technologies, the added third floor, and inadequate foundations, architectural and engineering investigations found that the Executive Residence portion of the White House Complex was facing near-imminent collapse, and it was deemed unsafe for occupancy. President Harry S. Truman, his family, and the entire residence staff were relocated across the street to Blair House, and over the next three years, the White House was gutted, expanded, and rebuilt.
https://en.wikipedia.org/wiki/White_House_Reconstruction
https://en.wikipedia.org/wiki/White_House_Reconstruction