A quotation that arrives before anyone has been on the roof is a guess in a nice typeface. A serious survey takes time and ends with the answers in hand rather than estimated.
Anchor points
The first thing checked on the roof: whether anchor points exist, where they are, and whether their condition allows them to be used. Their absence does not make the work impossible, but it changes the access method, the cost and the duration. That alone can rewrite a proposal.
The surface itself
Glass, cladding, stone or mesh, whether it carries a coated finish, and whether there are marks from a previous clean. An elevation that has already been cleaned with the wrong product gets handled more carefully, because the damage left behind is not ours and should be documented before we start.
The ground and what surrounds it
What sits beneath the elevation: parking, a footpath, the main entrance, trees. This decides whether a lift is an option at all, how much ground has to be cleared, and for how long.
How the building is used
When people arrive and leave, when the entrance is busiest, and whether there are days the building cannot lose anything at all. The schedule comes out of this: days or nights, and which elevation first.
What you receive afterwards
The access method for each elevation, the crew size, the number of days, and what closes and when. If a number arrives without those, the number was not built around your building.