Q. Can we see changing guard while visiting the State Rooms?
It is not possible to see Changing the Guard from the State Rooms or the gardens of Buckingham Palace.
The reason is that the State Apartments are mainly in the West wing of the Palace, while Changing the Guard takes place in the forecourt in front of the East wing.
With the average tour, of the State Rooms and gardens lasting around two hours and first admissions at 09:45, the answer, if you want to do both on the same day, is to visit Changing the Guard first.