Slate has a reputation for breaking when disturbed, and it is true that old slates are brittle. A careful roofer takes them off one at a time, stacks them, and puts them back or replaces the few that crack. Done properly, the roof around the rooflight looks as though it was always like that.
The list below gives the order for one rooflight in slate. For a plain single unit the roof stage should take no more than two working days, and the plastering inside is extra to that.
Why slate needs its own flashing
Slate is laid in overlapping courses, so the metal around a rooflight must slip under the course above and sit over the course below. The profile of the flashing is cut to the slate pattern. A flat apron laid over the top is not good enough; it will leak at the first heavy storm.
That is a detail the quote will show, and one you can ask about. If a contractor cannot explain how the flashing meets the slate, look elsewhere.
Before we begin, we confirm where the removed slates will be stored and which neighbours' access we might need. A small point, but one that avoids a conversation on the day.