Probably 50% of the construction is based on the project by Gaudì, which was lost.
It's almost completely a contemporary project.
https://hyperallergic.com/152158/has-gaudis-sagrada-familia-...
https://sagradafamilia.barcelona-tickets.com/sagrada-familia...
"Spanish Civil War: In 1936, during the civil unrest, anarchists broke into the cathedral and stole the plans, and destroyed the church's crypt. Work only resumed after the unrest ended in 1939. The informational remains were pieced together to make Sagrada Familia what it is today."