Spiral stairs usually cannot be placed under a floor edge due to headroom restrictions as you pass under the edge when you go down the stair.
As an example, imagine looking down from above on a spiral that is sitting next to the edge of a top floor. Its outline would be a circle next to a line that represented the top floor (Figure 1).
Figure 1
Now imagine the circle slid under the floor edge so you could step
directly from the top floor onto the spiral top tread (Figure 2).
Figure 2
With the spiral in this position, you would step down five times to
be at the point where you need to go under the floor edge (see numbers
1 thkrough 5 in Figure 2). Assuming 9" down each step* you would
only have descended 45" (5 x 9"). You would have to duck under
the floor edge to continue.
* National building codes allow spiral tread rises up to 9-1/2" to maximize headroom clearance.
Figure 3 shows the spiral set out from the floor edge with the top landing installed. The landing acts as a bridge from the floor to the top spiral tread. The landing allows you to go down further before you pass under its edge, increasing your headroom.
Figure 3
In this case you would have stepped down eight times, for a total
of 8 x 9" = 72". Codes actually require 78", but headroom
can be further increased by angling one side of the landing (see "Top
Landing Angles").
Another important function of the landing is that it anchors the top of the spiral to the top floor system. The landing deck (walking surface) acts to stiffen the spiral top against sideways movement.
Each Precision Pine interior spiral kit includes the top landing as well as horizontal handrails required to guard the open sides of the landing.