https://www.cnet.com/news/notre-dame-cathedral-fire-what-caused-it-and-what-happens-next/

Elements like firewalls and sprinkler systems were reportedly missing from Notre Dame's attic, where the fire burned, by choice. Electrical wiring reportedly wasn't allowed in the cathedral's attic to preserve its original design and to protect the lead ceiling's timber support beams. 

Valérie Pécresse, the president of the Île-de-France region in which Paris lies, confirmed Tuesday that the fire was an accident, though officials haven't elaborated on the exact cause. Paris police said it may be linked to the $6.8 million renovation efforts underway.