The old stock exchange was being renovated and had been shrouded in scaffolding and protective plastic covering.

https://en.wikipedia.org/wiki/Notre-Dame_fire

The fire has been compared to the similar 1992 Windsor Castle fire and the Uppark fire, among others,[114] and has raised old questions about the safety of similar structures and the techniques used to restore them.[114] Renovation increases fire risk, and a police source reported they are looking into whether such work had caused this incident.[19][115]

Renovation presents a fire risk from sparks, short-circuits, and heat from welding (roof repairs involved cutting, and soldering lead sheets resting on timber[19]). Normally, no electrical is allowed in the roof space because of the extreme fire risk.[24] The roof framing was made of very dry timber, often powdery with age.[19] After the fire, the architect responsible for fire safety at the cathedral acknowledged that the rate at which fire might spread had been underestimated, and experts said it was well known that a fire in the roof would be almost impossible to control.[27]

https://en.wikipedia.org/wiki/Uppark#Fire

On 30 August 1989 the building was devastated by a fire caused by a workman’s blowtorch whilst repairing lead flashing on the roof, just two days before the work was due to be completed.[10] The fire broke out during opening hours. Many works of art and pieces of furniture were carried out of the burning building by members of the Meade-Fetherstonehaugh family, National Trust staff and members of the public. Although the garret and first floors collapsed onto the lower floors and the garret and first-floor contents were lost completely, the floors largely fell clear of the ground-floor walls and much of the panelling and decoration survived.