The limit of the grave’s height and making one side higher than the other because the ground is low
20-1-2014 | IslamWeb
Question:
What is the limit of the grave’s height in centimeters? The grave is in an inclined area and, in order to level the grave, one side should be higher than the other.
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
It is favorable to raise the grave as much as a span higher than the ground according to jurists. An-Nawawi 
said: “It is favored to raise the grave a span higher than the ground according to the statement of Ash-Shaafi‘i and an agreement of the scholars from the same school of jurisprudence.”
Ibn Qudaamah said: “The grave should be raised a span higher than the ground in order to be recognized as a grave and thus protected (from being trodden on) and mercy is asked for its dweller. It is narrated by As-Saaji on the authority of Jaabir 
that the grave of the Prophet, sallallaahu ‘alayhi wa sallam, was raised a span above the ground.”
It is narrated on the authority of Al-Qaasim ibn Muhammad that he said: “I said to ‘Aa’ishah 
: 'O mother, show me the grave of the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, and his two Companions.' She showed me three graves which were neither very high (over the ground) nor very prominent, topped with red stones.” [Abu Daawood]
It is also unfavorable to raise it higher than its dust as is narrated on the authority of ‘Uqbah Ibn ‘Aamir that he said: “Do not make the dust in the grave more than what comes out of it when it is dug.” [Ahmad]
It is narrated by Al-Khallaal on the authority of ‘Jaabir that he said: “The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, forbade increasing in the grave more than the dust coming out from digging it.”
It is also unfavorable to raise the grave except a little in response to the statement of the Prophet, sallallaahu ‘alayhi wa sallam, to ‘Ali “Do not leave a statue but that you should deface it nor a very high grave but that you should level it with the ground.” [Muslim and others]
Being so, a span is the space between the tips of the little finger and the thumb in an open hand. It equals twelve fingers which is equivalent to 23.1 cm according to the definition given in Mu‘jam Lughat Al-Fuqahaa’.
There is also no harm to raise one side of the grave somewhat higher than the other, in order to level the grave.
Allaah Knows best.