Is it obligatory to give out a quarter of the rent of a house endowed as a charity, or does the grandfather's will to donate a quarter of his money not include the rent of this house?

A waqf (endowment) is an asset whose principal is held in perpetuity, meaning it cannot be sold, gifted, or inherited. This is based on the Prophet Muhammad's (peace be upon him) statement to Umar ibn al-Khattab (may Allah be pleased with him): "Endow its principal so that it is not sold, gifted, or inherited, but its fruits are spent." Therefore, the endowed house is not included in your grandfather's will because it is not part of the inheritance.