Additional area is not the only change
The obvious effect of expansion is more floor to cover. The less obvious effects are usually more significant. A second entry brings soil into a part of the premises that previously had none. An additional kitchen or washroom adds facilities that need their own frequency. A new meeting room changes where visitors go, which changes which areas are front of house.
Movement patterns inside the original space also change. A corridor that was a dead end becomes a thoroughfare, an area that was peripheral becomes central, and floor wear redistributes accordingly. None of that is captured by adding a percentage to the existing hours, which is why a walkthrough of the whole tenancy rather than the new portion is the right starting point.
