Is it permissible to construct a residential building on land that contains graves, and if not permissible, is it then permissible to enter this building?

If the building is in a cemetery endowed for the burial of the dead, then it is forbidden and must be demolished. It is not permissible for Muslims to approve of it or sit in it. However, if the building is on his own property, he has done wrong, but he is not obligated to demolish it, and there is no harm in visiting him and sitting with him there.