When I went to the Holy Land in 2000, my favorite of the Masses celebrated each day of the pilgrimage was the outdoor Mass held at Shepherds’ Field near Bethlehem. I have also had the joy of participating in Masses occasionally celebrated in my home. So, yes, there are conditions in which outdoor Masses or home Masses are appropriate.
That said, care should be taken not to cause scandal. There should either be some reason why the Mass is being held outdoors (e.g., no churches nearby, the site itself is of religious importance, etc.) or the Mass should be private and not be publicized to the larger community (e.g., a Mass offered privately in a home by a priest who is a personal friend). As a matter of ordinary protocol, the Church expects Mass to be held in a church consecrated to the worship of God.
