The White House was gutted for renovation during the Truman years, but the furnishings were removed first.
But the interior walls weren't necessarily rebuilt to the same plan as the original walls. The new building has a steel frame to support the walls and floors. This means the exterior walls are no longer load bearing, and the interior walls could be mad much thinner than before. This increased the amount of space available while allowing modern conveniences like electricity, plumbing, telecommunications wires, and air conditioning to be built in.