It is disliked for the imam to be at a higher place than the followers. However, it is permissible for the followers to be at a higher place than the imam.
Sheikh Sayyed Sabiq states in his well-known book, Fiqh Us-Sunnah: Abu Mas`ud Al-Ansari says, “The Prophet (peace and blessings be upon him prohibited that the imam should stand on something higher than the people behind him” (Ad-Daraqutni). Hamam ibn Al-Harith relates that Hudhayfah led the people in Prayer in Iraq and he stood on a bench. Abu Mas`ud pulled his shirt with a strong grip. When he finished his Prayer Abu Mas`ud said, “Do you not know that this has been prohibited?” Hudhayfah said, “Certainly, I know it. I remembered it when you pulled me” (Abu Dawud, Ash-Shafi`i, and Al-Bayhaqi).
On the other hand, if the imam has some reason for being higher than the followers, the act is not disliked. Sahl ibn Sa`d As-Sa`idy says, “I saw the Prophet sitting upon the pulpit on the first day that it was set up. He made the opening takbir while he was upon it and then he performed ruku` (bowing). Afterward, he moved behind the pulpit and made sajdah (prostration) at the foot of the pulpit. Then, he repeated the same. When he had finished, he turned to the people and said, ‘O people, I did that for you to follow me and to teach you my Prayer’“ (Ahmad, Al-Bukhari, and Muslim).
However, It is permissible for the followers to be at a higher place than the imam. Sa`id ibn Mansur, Ash-Shaf`i, Al-Bayhaqi, and Al-Bukhari, in his comments, relate from Abu Hurayrah that he prayed at the top of the mosque while following the imam. Anas used to pray in the room of Abu Nafi` to the right of the mosque and the [floor of the] room was as high as his height [above the mosque] and its door faced the mosque of Basrah and Anas would pray in it, following the imam. The Companions did not say anything about it. Ash-Shawkani observes, “If the follower is extremely high above the imam, for example, three hundred lengths, and he could not know what action the imam is doing, then it is prohibited by consensus whether he is in a mosque or somewhere else. If it is less than that, it is permitted on the principle that unless proved otherwise a thing is permissible. This basis is supported by the above mentioned act of Abu Hurayrah, to which no one objected.”