Leonardo's early painting of the Annunciation owes much to the influence of his master, Andrea del Verrocchio. However it is considered to be da Vinci's first major work, a large painting executed by his own hand with, perhaps, the help of Verrocchio's workshop. In fact the attribution to Leonardo was only presented in 1867 and the debates about the parts actually painted by him continued for a long time.
Importantly certain aspects of the painting such as the composition, the Archangel Gabriel and the landscapes are pure da Vinci but agreement on many aspects of attributions remain far from unanimous. Some details of the painting, such as the wings of the angel, have been repainted by another hand and the marble sarcophagus has been copied from a carving by Leonardo's master, Andrea del Verrocchio. For several years the painting was thought to be the work of Domenico Ghirlandaio, or Lorenzo di Credi, Credi was apprenticed to Verrocchio's workshop at the same time as Leonardo.
It is feasable that the work is a colaboration between several artists with the finishing touches completed by Leonardo.