Basement Subfloor at Bottom Stair Landing
New landing at height of last step

Build the new basement floor across the full room.
Add a new landing - really just another floor that is at the height of the existing bottom step. Depending on how you finish it, you may add a thin (max. 3/8&quot; ideally, as already noted) layer on top of both the stair and the new floor to make it function and appear as one smooth surface. This landing should be at least 3 feet deep, though that includes the stair that becomes part of it.

That leaves you with one short step at the end of the landing down to the new basement floor.