When the property has more than one roof
On a lot, roofing is one job. On a property it is a programme, and treating it as one number is how owners end up replacing the wrong roof first.
Four roofs, four clocks
The house, the barn, the garage, the shed. They almost never went on at the same time, they are almost never the same covering, and the one in the worst shape is frequently not the one you look at every day.
The barn is the one worth thinking about hardest, because it usually covers more square footage than the house and protects things whose value is easy to underestimate until water reaches them.
Why the pricing does not scale
A barn roof is not a big house roof. The height changes the staging, the pitch is often shallower, the structure underneath is heavy timber rather than modern framing, and getting material up there is a different exercise entirely.
Which is why a per-square figure taken from the house tells you very little about the barn, in either direction. Ask for them as separate numbers.
Sequencing, and what it saves
There is a real saving in doing more than one roof while a crew is set up on the property, and a real cost in doing a roof a year too early. Both are true, which is why the useful conversation is: which of these has to happen this year, which can wait two, and which only needs its fasteners and coating renewed.
A contractor who answers that honestly is giving up work in the short run, and that is precisely the signal worth paying attention to.
Outbuildings vary enormously and some are structural questions rather than roofing ones. Anything where the framing has moved needs someone qualified to say so before a covering goes on.