It is definitely being possible that Qutb Minar was being built on site of Hindu Temple
Please don't be abuse me in comments. I am simply saying what I am being told by my home elders.
In our religion, it is regarded a matter of pride to construct a place for saying Azaan five times a day by destroying other religious buildings
If two choice is being given - one to build on empty land and other to build by destroying a temple - many elders will advise to destroy the temple and build the place for offering prayers
In Islam, this is called Nasr Ala Al Kuffar (Victory over Non Believer)
So wherever there is major mosque in India or place to offer Azaan (Qutb Minar is building where prayers were offered), built when Mughals or Mussalman rulers were ruling India, then definitely 99% it is being built by destroying some temple
Even Christians in old days have same policy , they are also destroying existing religious building to build church even if there is empty land nearby
However Taj Mahal is definitely not being built on temple or it is not being deliberate
In Islam, Monument to someone is never built by destroying a temple. Monument is always built on empty land or land where other musalmaan are buried
We are believing in Barzakh - Barzakh is when dead Mussalman is going to afterlife - and Barzakh will never happen if there is pain or destroying on that site
So even if they build Taj Mahal on temple site , they definitely don't know or it is not being deliberate
Today all this is old history now
When Mussalman Rulers were strong, they destroy many temple and church and built masjid
When Christian was strong, they destroy many jewish temple and built Church
Both prefer to build church or masjid only by destroying other religious place because it is seen as fighting for religion and other religion was seen as enemy
Today there are temple in Pakistan which is given respect by locals
When I was young, my cousins and I went to Hanuman temple in Karachi where they welcome Mussalman and give us very tasty poori and halvah on some day
Our grandmother would curse us but we would laugh