Building mausoleums are allowed in Islam...How will you justify all these Ahadith !!
1) It is in Hadith - Narrated Abu Bakr bin 'Aiyash (رضئ اللہ تعالی عنہ): Sufyan at-Tammar (رضئ اللہ تعالی عنہ) told me that he had seen the Grave of the Prophet (صلى الله عليه و آله وسلم)elevated and convex. (Bukhari - Book #23, Hadith #473).
All Muslims of the world know that the three pious graves inside Masjid-e-Nabawi (صلى الله عليه و آله وسلم) are elevated, considerably higher from the ground. These remain the same from the earliest times as reported in the above Hadith.
(2) It is in Hadith - Narrated Al-Qasim ibn Muhammad ibn AbuBakr(رضئ اللہ تعالی عنہ) : I said to Ummul Momineen Aisha (رضئ اللہ تعالی عنہا)! Mother, show me the Grave of the Apostle of Allah (صلى الله عليه و آله وسلم) and his two Companions (رضئ اللھ تعالی عنہم). She showed me three Graves which were neither high nor low, but were spread with soft red pebbles in an open space (in the secured room). (Abu Dawood Book #20, Hadith #3214).
The above Hadith nails the lies of Wahhabis / Salafis / Deobandis and their like minded groups. The Wahhabis have destroyed and flattened the graves (without a trace) of all family members of Prophet Mohammad(صلى الله عليه و آله وسلم) Sahabah(رضئ اللھ تعالی عنہم اجمعین ), Imams and Awliya Allah. There were hundreds of thousands pious graves of Sahabah in Arabian peninsula. Now, there is none available.
There is no Hadith or Quranic Verse which saysthat the Graves of the family members of Prophet Mohammad (صلى الله عليه و آله وسلم), Sahabah(رضئ اللھ تعالی عنہم اجمعین ) should be destroyed and flattened to the ground without a trace.
(3) It is in Hadith - Narrated by Abu Dawood and al-Bayhaqi in al-Kubra (3:412) with fair chains. Ibn Hajar, Talkhis al-Habir (2:134) andIbn al-Mulaqqin, Tuhfat al-Muhtaj (2:29): "Prophet (صلى الله عليه و آله وسلم)asked a man (Sahabi) to place a rock on top of Ibn Maz`un's (رضئ اللہ تعالی عنہ) Grave. When he was unable to move it (as the rock was too large), Allah's Apostle (صلى الله عليه و آله وسلم) rolled up his sleeves and helped him and the whiteness of his arms was visible (to everyone). Ibn Maz`un (رضئ اللہ تعالی عنہ) was the first of the Muhajirin buried in Baqi al-Gharqad. Hadhrat Ibrahim (رضئ اللہ تعالی عنہ), the Prophet's (صلى الله عليه و آله وسلم) son, was buried next to him. (Abu Dawood, al-Bayhaqi, Ibn Hajar, Ibn al-Mulaqqin).
The above Hadith was also mentioned by Bukhari under Chapter titled 'Stalk on the top of the Grave'.
Ibn Hajar said in Fath al-Bari (3:256=1959 ed. 3:223): "Al-Bukhari narrated this Hadith with its (sound) chain in al-Tarikh al-Saghir (1:42) "
The above Hadith clearly establishes the fact that it is Sunnah to make the graves of Sahabah and Awliya Allah higher and elevated from the ground and prominent to keep them as memorials for the following generations. It is also important to visit these pious graves for Dua. If you remove these important memorials, then you will deprive the entire Ummah of the Dua of these pious souls.