The side rooms were on three levels, one above another, thirty on each level. There were ledges all around the wall of the temple to serve as supports for the side rooms, so that the supports were not inserted into the wall of the temple.
References for Ezekiel 41:6
The side rooms all around the temple were wider at each successive level. The structure surrounding the temple was built in ascending stages, so that the rooms widened as one went upward. A stairway went up from the lowest floor to the top floor through the middle floor.
References for Ezekiel 41:7
I saw that the temple had a raised base all around it, forming the foundation of the side rooms. It was the length of the rod, six long cubits.
The outer wall of the side rooms was five cubits thick. The open area between the side rooms of the temple
and the priests’ rooms was twenty cubits wide all around the temple.
There were entrances to the side rooms from the open area, one on the north and another on the south; and the base adjoining the open area was five cubits wide all around.