The area that suffers is the old one
When an area is added without time being added, the crew does not simply work faster. They shorten something, and what gets shortened is whatever they judge least likely to be noticed: the back stairs, the storeroom, the detail work in an office nobody visits. The new area is done properly because it is new and visible, and an established part of the building declines without any decision having been made.
That decline is slow enough to be attributed to something else. By the time anybody notices, the addition happened three months earlier and nobody connects the two. Which is why the assessment of the additional time is done before the first shift rather than being tested by seeing whether the crew copes, since coping is exactly what a good crew will appear to do. Adding the time first is cheaper than diagnosing a decline three months later.
