A Window Is a Water Detail, Not a Glass Order
The glass unit is the easy part. What earns its keep is everything at the edges: the sill, the flashing, the water-resistive barrier and how the siding and trim close back over them. Set a modest window into a clean, flashed opening and it performs. Set a good one into a sloppy opening and you find out the first wet winter.
Where two jobs split
The biggest fork is whether the unit drops into the existing frame or the opening goes back to the studs. The first keeps the wall mostly intact. The second is the call when the old frame, sill or trim should not stay buried behind new glass, and it is usually what moves the price.
Old Seattle openings are rarely square
Older homes move. Headers sag a little, sills tip, and a unit ordered to the listed size will not sit right without shimming and trim work. We measure each opening on its own instead of trusting one number for the whole house.
When the window is really another job
A bedroom window can be an egress question, which ties into basement finishing. A wall full of windows usually overlaps siding work. A roof opening is a skylight and lands on the roofing side. And windows and doors share walls, trim and orders often enough that we plan them together.
The honest version: we price the opening and the water path, not just the unit, so the number still means something in February.