Can I state in my Will that a certain property belonging to me will be Waqf for the Musjid, but while I am alive, I shall derive the benefit of the ho

Answer

In this case, if the value of the house does not exceed one third the total value of your estate, the Waqf of the house will be valid. However, if its value is more than one third your estate, then the Waqf will be valid in only one third the value of your estate.

You can however make the house Waqf during your lifetime with the stipulation that whilst you are alive all proceeds and benefits of the land will be for you. You should not suspend the Waqf with your death.