What is the biblical basis for the notional/ideal pre-existence of Jesus, as opposed to an actual pre-existence?

Some Christians -- especially Biblical Unitarians -- believe that Jesus only pre-existed notionally/ideally in God's mind prior to his conception in Mary's womb (just like any other human being), contrary to more popular views such as that Jesus has existed since eternity (the view of most Christians) or since a long time ago (the view of Jehovah's Witnesses). What is the biblical basis for this view?